diff --git a/.github/workflows/check.yml b/.github/workflows/check.yml
index 9cfcd6cc..34b1e30b 100644
--- a/.github/workflows/check.yml
+++ b/.github/workflows/check.yml
@@ -32,8 +32,9 @@ jobs:
         run: |
           mkdir junit
           bundle exec parallel_rspec --prefix-output-with-test-env-number --first-is-1 -- spec/${{ matrix.type }}
-      - uses: actions/upload-artifact@v3
+      - uses: actions/upload-artifact@v4
         with:
+          name: ${{ matrix.ruby }}-${{ matrix.protocol }}-${{ matrix.type }}
           path: |
             junit/
             coverage/
diff --git a/.rubocop.yml b/.rubocop.yml
new file mode 100644
index 00000000..55801c59
--- /dev/null
+++ b/.rubocop.yml
@@ -0,0 +1,2 @@
+AllCops:
+  DisabledByDefault: true
diff --git a/lib/ably/modules/ably.rb b/lib/ably/modules/ably.rb
index 0cb5ef9a..1a746dbd 100644
--- a/lib/ably/modules/ably.rb
+++ b/lib/ably/modules/ably.rb
@@ -4,19 +4,26 @@
 #
 # @see file:README.md README
 module Ably
-  # Fallback hosts to use when a connection to rest/realtime.ably.io is not possible due to
+  # Fallback hosts to use when a connection to main.realtime.ably.net is not possible due to
   # network failures either at the client, between the client and Ably, within an Ably data center, or at the IO domain registrar
   # see https://ably.com/docs/client-lib-development-guide/features/#RSC15a
   #
-  FALLBACK_DOMAIN = 'ably-realtime.com'.freeze
+  PROD_FALLBACK_DOMAIN = 'ably-realtime.com'.freeze
+  NONPROD_FALLBACK_DOMAIN = 'ably-realtime-nonprod.com'.freeze
+
   FALLBACK_IDS = %w(a b c d e).freeze
 
-  # Default production fallbacks a.ably-realtime.com ... e.ably-realtime.com
-  FALLBACK_HOSTS = FALLBACK_IDS.map { |host| "#{host}.#{FALLBACK_DOMAIN}".freeze }.freeze
+  # Default production fallbacks main.a.fallback.ably-realtime.com ... main.e.fallback.ably-realtime.com
+  FALLBACK_HOSTS = FALLBACK_IDS.map { |host| "main.#{host}.fallback.#{PROD_FALLBACK_DOMAIN}".freeze }.freeze
+
+  # Prod fallback suffixes a.fallback.ably-realtime.com ... e.fallback.ably-realtime.com
+  PROD_FALLBACKS_SUFFIXES = FALLBACK_IDS.map do |host|
+    "#{host}.fallback.#{PROD_FALLBACK_DOMAIN}".freeze
+  end.freeze
 
-  # Custom environment default fallbacks {ENV}-a-fallback.ably-realtime.com ... {ENV}-a-fallback.ably-realtime.com
-  CUSTOM_ENVIRONMENT_FALLBACKS_SUFFIXES = FALLBACK_IDS.map do |host|
-    "-#{host}-fallback.#{FALLBACK_DOMAIN}".freeze
+  # Nonprod fallback suffixes a.fallback.ably-realtime-nonprod.com ... e.fallback.ably-realtime-nonprod.com
+  NONPROD_FALLBACKS_SUFFIXES = FALLBACK_IDS.map do |host|
+    "#{host}.fallback.#{NONPROD_FALLBACK_DOMAIN}".freeze
   end.freeze
 
   INTERNET_CHECK = {
diff --git a/lib/ably/realtime/client.rb b/lib/ably/realtime/client.rb
index b35aaa94..60e2333b 100644
--- a/lib/ably/realtime/client.rb
+++ b/lib/ably/realtime/client.rb
@@ -14,8 +14,6 @@ class Client
       extend Forwardable
       using Ably::Util::AblyExtensions
 
-      DOMAIN = 'realtime.ably.io'
-
       # A {Aby::Realtime::Channels} object.
       #
       # @spec RTC3, RTS1
@@ -73,7 +71,7 @@ class Client
       def_delegators :auth, :client_id, :auth_options
       def_delegators :@rest_client, :encoders
       def_delegators :@rest_client, :use_tls?, :protocol, :protocol_binary?
-      def_delegators :@rest_client, :environment, :custom_host, :custom_port, :custom_tls_port
+      def_delegators :@rest_client, :endpoint, :environment, :custom_host, :custom_port, :custom_tls_port
       def_delegators :@rest_client, :log_level
       def_delegators :@rest_client, :options
 
@@ -289,10 +287,34 @@ def publish(channel_name, name, data = nil, attributes = {}, &success_block)
         end
       end
 
-      # @!attribute [r] endpoint
+      # @!attribute [r] hostname
+      # @return [String] The primary hostname to connect to Ably
+      def hostname
+        if endpoint.include?('.') || endpoint.include?('::') || endpoint == 'localhost'
+          return endpoint
+        end
+
+        if endpoint.start_with?('nonprod:')
+          "#{endpoint.gsub('nonprod:', '')}.realtime.#{root_domain}"
+        else
+          "#{endpoint}.realtime.#{root_domain}"
+        end
+      end
+
+      # @!attribute [r] root_domain
+      # @return [String] The root domain used in the hostname
+      def root_domain
+        if endpoint.start_with?('nonprod:')
+          'ably-nonprod.net'
+        else
+          'ably.net'
+        end
+      end
+
+      # @!attribute [r] uri
       # @return [URI::Generic] Default Ably Realtime endpoint used for all requests
-      def endpoint
-        endpoint_for_host(custom_realtime_host || [environment, DOMAIN].compact.join('-'))
+      def uri
+        uri_for_host(custom_realtime_host || hostname)
       end
 
       # (see Ably::Rest::Client#register_encoder)
@@ -322,8 +344,8 @@ def disable_automatic_connection_recovery
       # @api private
       def fallback_endpoint
         unless defined?(@fallback_endpoints) && @fallback_endpoints
-          @fallback_endpoints = fallback_hosts.shuffle.map { |fallback_host| endpoint_for_host(fallback_host) }
-          @fallback_endpoints << endpoint # Try the original host last if all fallbacks have been used
+          @fallback_endpoints = fallback_hosts.shuffle.map { |fallback_host| uri_for_host(fallback_host) }
+          @fallback_endpoints << uri # Try the original host last if all fallbacks have been used
         end
 
         fallback_endpoint_index = connection.manager.retry_count_for_state(:disconnected) + connection.manager.retry_count_for_state(:suspended) - 1
@@ -341,7 +363,8 @@ def device
       end
 
       private
-      def endpoint_for_host(host)
+
+      def uri_for_host(host)
         port = if use_tls?
           custom_tls_port
         else
diff --git a/lib/ably/realtime/connection.rb b/lib/ably/realtime/connection.rb
index 6295ef09..653f764b 100644
--- a/lib/ably/realtime/connection.rb
+++ b/lib/ably/realtime/connection.rb
@@ -173,7 +173,7 @@ def initialize(client, options)
         @state         = STATE(state_machine.current_state)
         @manager       = ConnectionManager.new(self)
 
-        @current_host = client.endpoint.host
+        @current_host = client.uri.host
 
         reset_client_msg_serial
       end
@@ -396,12 +396,13 @@ def determine_host
             @current_host = if internet_is_up_result
               client.fallback_endpoint.host
             else
-              client.endpoint.host
+              client.uri.host
             end
             yield current_host
           end
         else
-          @current_host = client.endpoint.host
+          # Use hostname from client.uri (#REC1)
+          @current_host = client.uri.host
           yield current_host
         end
       end
@@ -496,8 +497,8 @@ def create_websocket_transport
                 end
               end
 
-              url = URI(client.endpoint).tap do |endpoint|
-                endpoint.query = URI.encode_www_form(url_params)
+              url = URI(client.uri).tap do |uri|
+                uri.query = URI.encode_www_form(url_params)
               end
 
               determine_host do |host|
diff --git a/lib/ably/rest/client.rb b/lib/ably/rest/client.rb
index 3dbda2df..c37aeef6 100644
--- a/lib/ably/rest/client.rb
+++ b/lib/ably/rest/client.rb
@@ -18,9 +18,6 @@ class Client
       extend Forwardable
       using Ably::Util::AblyExtensions
 
-      # Default Ably domain for REST
-      DOMAIN = 'rest.ably.io'
-
       MAX_MESSAGE_SIZE = 65536 # See spec TO3l8
       MAX_FRAME_SIZE = 524288 # See spec TO3l8
 
@@ -43,7 +40,11 @@ class Client
 
       def_delegators :auth, :client_id, :auth_options
 
-      # Custom environment to use such as 'sandbox' when testing the client library against an alternate Ably environment
+      # The hostname used to connect to Ably
+      # @return [String]
+      attr_reader :endpoint
+
+      # Custom environment to use such as 'sandbox' when testing the client library against an alternate Ably environment (deprecated)
       # @return [String]
       attr_reader :environment
 
@@ -135,7 +136,8 @@ class Client
       # @option options [String]                  :token               Token string or {Models::TokenDetails} used to authenticate requests
       # @option options [String]                  :token_details       {Models::TokenDetails} used to authenticate requests
       # @option options [Boolean]                 :use_token_auth      Will force Basic Auth if set to false, and Token auth if set to true
-      # @option options [String]                  :environment         Specify 'sandbox' when testing the client library against an alternate Ably environment
+      # @option options [String]                  :endpoint            Specify a routing policy or fully-qualified domain name to connect to Ably
+      # @option options [String]                  :environment         Specify 'sandbox' when testing the client library against an alternate Ably environment (deprecated)
       # @option options [Symbol]                  :protocol            (:msgpack) Protocol used to communicate with Ably, :json and :msgpack currently supported
       # @option options [Boolean]                 :use_binary_protocol (true) When true will use the MessagePack binary protocol, when false it will use JSON encoding. This option will overide :protocol option
       # @option options [Logger::Severity,Symbol] :log_level           (Logger::WARN) Log level for the standard Logger that outputs to STDOUT. Can be set to :fatal (Logger::FATAL), :error (Logger::ERROR), :warn (Logger::WARN), :info (Logger::INFO), :debug (Logger::DEBUG) or :none
@@ -188,8 +190,6 @@ def initialize(options)
         @agent               = options.delete(:agent) || Ably::AGENT
         @realtime_client     = options.delete(:realtime_client)
         @tls                 = options.delete_with_default(:tls, true)
-        @environment         = options.delete(:environment) # nil is production
-        @environment         = nil if [:production, 'production'].include?(@environment)
         @protocol            = options.delete(:protocol) || :msgpack
         @debug_http          = options.delete(:debug_http)
         @log_level           = options.delete(:log_level) || ::Logger::WARN
@@ -203,9 +203,14 @@ def initialize(options)
         @max_frame_size      = options.delete(:max_frame_size) || MAX_FRAME_SIZE
         @idempotent_rest_publishing = options.delete_with_default(:idempotent_rest_publishing, true)
 
+        @environment         = options.delete(:environment) # nil is production
+        @environment         = nil if [:production, 'production'].include?(@environment)
+        @endpoint            = environment || options.delete_with_default(:endpoint, 'main')
+
         if options[:fallback_hosts_use_default] && options[:fallback_hosts]
           raise ArgumentError, "fallback_hosts_use_default cannot be set to try when fallback_hosts is also provided"
         end
+
         @fallback_hosts = case
         when options.delete(:fallback_hosts_use_default)
           Ably::FALLBACK_HOSTS
@@ -213,8 +218,10 @@ def initialize(options)
           options_fallback_hosts
         when custom_host || options[:realtime_host] || custom_port || custom_tls_port
           []
-        when environment
-          CUSTOM_ENVIRONMENT_FALLBACKS_SUFFIXES.map { |host| "#{environment}#{host}" }
+        when endpoint.start_with?('nonprod:')
+          NONPROD_FALLBACKS_SUFFIXES.map { |host| "#{endpoint.gsub('nonprod:', '')}.#{host}" }
+        when endpoint != 'main'
+          PROD_FALLBACKS_SUFFIXES.map { |host| "#{endpoint}.#{host}" }
         else
           Ably::FALLBACK_HOSTS
         end
@@ -426,10 +433,34 @@ def push
         @push ||= Push.new(self)
       end
 
-      # @!attribute [r] endpoint
+      # @!attribute [r] hostname
+      # @return [String] The primary hostname to connect to Ably
+      def hostname
+        if endpoint.include?('.') || endpoint.include?('::') || endpoint == 'localhost'
+          return endpoint
+        end
+
+        if endpoint.start_with?('nonprod:')
+          "#{endpoint.gsub('nonprod:', '')}.realtime.#{root_domain}"
+        else
+          "#{endpoint}.realtime.#{root_domain}"
+        end
+      end
+
+      # @!attribute [r] root_domain
+      # @return [String] The root domain used in the hostname
+      def root_domain
+        if endpoint.start_with?('nonprod:')
+          'ably-nonprod.net'
+        else
+          'ably.net'
+        end
+      end
+
+      # @!attribute [r] uri
       # @return [URI::Generic] Default Ably REST endpoint used for all requests
-      def endpoint
-        endpoint_for_host(custom_host || [@environment, DOMAIN].compact.join('-'))
+      def uri
+        uri_for_host(custom_host || hostname)
       end
 
       # @!attribute [r] logger
@@ -480,7 +511,7 @@ def connection(options = {})
         if options[:use_fallback]
           fallback_connection
         else
-          @connection ||= Faraday.new(endpoint.to_s, connection_options)
+          @connection ||= Faraday.new(uri.to_s, connection_options)
         end
       end
 
@@ -493,7 +524,7 @@ def connection(options = {})
       # @api private
       def fallback_connection
         unless defined?(@fallback_connections) && @fallback_connections
-          @fallback_connections = fallback_hosts.shuffle.map { |host| Faraday.new(endpoint_for_host(host).to_s, connection_options) }
+          @fallback_connections = fallback_hosts.shuffle.map { |host| Faraday.new(uri_for_host(host).to_s, connection_options) }
         end
         @fallback_index ||= 0
 
@@ -653,7 +684,7 @@ def reauthorize_on_authorization_failure
         end
       end
 
-      def endpoint_for_host(host)
+      def uri_for_host(host)
         port = if use_tls?
           custom_tls_port
         else
diff --git a/spec/acceptance/realtime/client_spec.rb b/spec/acceptance/realtime/client_spec.rb
index b392d118..6c67b855 100644
--- a/spec/acceptance/realtime/client_spec.rb
+++ b/spec/acceptance/realtime/client_spec.rb
@@ -34,7 +34,7 @@
             client.connection.once(:failed) do
               expect(custom_logger_object.logs.find do |severity, message|
                 next unless %w(fatal error).include?(severity.to_s)
-                message.match(%r{https://help.ably.io/error/40400})
+                message.match(%r{https://help.ably.io/error/40101})
               end).to_not be_nil
               stop_reactor
             end
@@ -234,7 +234,7 @@
     context '#request (#RSC19*)' do
       let(:client_options) { default_options.merge(key: api_key) }
       let(:device_id) { random_str }
-      let(:endpoint) { subject.rest_client.endpoint }
+      let(:uri) { subject.rest_client.uri }
 
       context 'get' do
         it 'returns an HttpPaginatedResponse object' do
@@ -287,7 +287,7 @@
 
       context 'post', :webmock do
         before do
-          stub_request(:delete, "#{endpoint}/push/deviceRegistrations/#{device_id}/resetUpdateToken").
+          stub_request(:delete, "#{uri}/push/deviceRegistrations/#{device_id}/resetUpdateToken").
             to_return(status: 200, body: '{}', headers: { 'Content-Type' => 'application/json' })
         end
 
@@ -301,7 +301,7 @@
 
       context 'delete', :webmock do
         before do
-          stub_request(:delete, "#{endpoint}/push/channelSubscriptions?deviceId=#{device_id}").
+          stub_request(:delete, "#{uri}/push/channelSubscriptions?deviceId=#{device_id}").
             to_return(status: 200, body: '{}', headers: { 'Content-Type' => 'application/json' })
         end
 
@@ -317,7 +317,7 @@
         let(:body_params) { { 'metadata' => { 'key' => 'value' } } }
 
         before do
-          stub_request(:patch, "#{endpoint}/push/deviceRegistrations/#{device_id}")
+          stub_request(:patch, "#{uri}/push/deviceRegistrations/#{device_id}")
             .with(body: serialize_body(body_params, protocol))
             .to_return(status: 200, body: '{}', headers: { 'Content-Type' => 'application/json' })
         end
@@ -341,7 +341,7 @@
         end
 
         before do
-          stub_request(:put, "#{endpoint}/push/deviceRegistrations/#{device_id}")
+          stub_request(:put, "#{uri}/push/deviceRegistrations/#{device_id}")
             .with(body: serialize_body(body_params, protocol))
             .to_return(status: 200, body: '{}', headers: { 'Content-Type' => 'application/json' })
         end
diff --git a/spec/acceptance/realtime/connection_failures_spec.rb b/spec/acceptance/realtime/connection_failures_spec.rb
index 6afb7616..7f4de0cb 100644
--- a/spec/acceptance/realtime/connection_failures_spec.rb
+++ b/spec/acceptance/realtime/connection_failures_spec.rb
@@ -31,8 +31,8 @@
               error = connection_state_change.reason
               expect(connection.state).to eq(:failed)
               # TODO: Check error type is an InvalidToken exception
-              expect(error.status).to eq(404)
-              expect(error.code).to eq(40400) # not found
+              expect(error.status).to eq(401)
+              expect(error.code).to eq(40101) # not found
               stop_reactor
             end
           end
@@ -46,7 +46,7 @@
               error = connection_state_change.reason
               expect(connection.state).to eq(:failed)
               # TODO: Check error type is a TokenNotFound exception
-              expect(error.status).to eq(401)
+              expect(error.status).to eq(404)
               expect(error.code).to eq(40400) # not found
               stop_reactor
             end
@@ -930,7 +930,7 @@ def send_disconnect_message
                   original_method.call(*args, &block)
                 end
                 connection.once(:connected) do
-                  host = "#{"#{environment}-" if environment && environment.to_s != 'production'}#{Ably::Realtime::Client::DOMAIN}"
+                  host = client.hostname
                   expect(hosts.first).to eql(host)
                   expect(hosts.length).to eql(1)
                   stop_reactor
@@ -1396,10 +1396,10 @@ def kill_connection_transport_and_prevent_valid_resume
           channel = client.channels.get("foo")
           channel.attach do
             connection.once(:failed) do |state_change|
-              expect(state_change.reason.code).to eql(40400)
-              expect(connection.error_reason.code).to eql(40400)
+              expect(state_change.reason.code).to eql(40101)
+              expect(connection.error_reason.code).to eql(40101)
               expect(channel).to be_failed
-              expect(channel.error_reason.code).to eql(40400)
+              expect(channel.error_reason.code).to eql(40101)
               stop_reactor
             end
 
@@ -1469,13 +1469,13 @@ def kill_connection_transport_and_prevent_valid_resume
       end
 
       context 'with non-production environment' do
-        let(:environment)    { 'sandbox' }
-        let(:expected_host)  { "#{environment}-#{Ably::Realtime::Client::DOMAIN}" }
+        let(:environment)    { 'nonprod:sandbox' }
+        let(:expected_host)  { "sandbox.realtime.ably-nonprod.net" }
         let(:client_options) { timeout_options.merge(environment: environment) }
 
         context ':fallback_hosts_use_default is unset' do
           let(:max_time_in_state_for_tests) { 8 }
-          let(:expected_hosts) { Ably::CUSTOM_ENVIRONMENT_FALLBACKS_SUFFIXES.map { |suffix| "#{environment}#{suffix}" } + [expected_host] }
+          let(:expected_hosts) { Ably::NONPROD_FALLBACKS_SUFFIXES.map { |suffix| "#{environment.gsub("nonprod:", "")}.#{suffix}" } + [expected_host] }
           let(:fallback_hosts_used) { Array.new }
 
           it 'uses fallback hosts by default' do
@@ -1568,7 +1568,7 @@ def kill_connection_transport_and_prevent_valid_resume
           stub_const 'Ably::FALLBACK_HOSTS', custom_hosts
         end
 
-        let(:expected_host)  { Ably::Realtime::Client::DOMAIN }
+        let(:expected_host)  { 'main.realtime.ably.net' }
         let(:client_options) { timeout_options.merge(environment: nil) }
 
         let(:fallback_hosts_used) { Array.new }
diff --git a/spec/acceptance/realtime/connection_spec.rb b/spec/acceptance/realtime/connection_spec.rb
index 36ca720f..c4099027 100644
--- a/spec/acceptance/realtime/connection_spec.rb
+++ b/spec/acceptance/realtime/connection_spec.rb
@@ -28,8 +28,8 @@
       end
 
       context 'current_host' do
-        it 'is available immediately after the client is instanced' do
-          expect(connection.current_host.to_s).to match(/\.ably\.io$/)
+        it 'is available immediately after the client is instanced (#REC1)' do
+          expect(connection.current_host.to_s).to match(/\.realtime\.ably[-nonprod]*\.net$/)
           stop_reactor
         end
       end
diff --git a/spec/acceptance/realtime/message_spec.rb b/spec/acceptance/realtime/message_spec.rb
index b218a8e1..63c8aa35 100644
--- a/spec/acceptance/realtime/message_spec.rb
+++ b/spec/acceptance/realtime/message_spec.rb
@@ -775,7 +775,7 @@ def publish_and_check_extras(extras)
                 EventMachine.add_timer(0.0001) do
                   connection.transition_state_machine :suspended
                   stub_const 'Ably::FALLBACK_HOSTS', []
-                  allow(client).to receive(:endpoint).and_return(URI::Generic.build(scheme: 'wss', host: 'does.not.exist.com'))
+                  allow(client).to receive(:uri).and_return(URI::Generic.build(scheme: 'wss', host: 'does.not.exist.com'))
                 end
               end
             end
diff --git a/spec/acceptance/realtime/push_admin_spec.rb b/spec/acceptance/realtime/push_admin_spec.rb
index e9dfba92..1be57968 100644
--- a/spec/acceptance/realtime/push_admin_spec.rb
+++ b/spec/acceptance/realtime/push_admin_spec.rb
@@ -102,7 +102,7 @@
         end
 
         let!(:publish_stub) do
-          stub_request(:post, "#{client.rest_client.endpoint}/push/publish").
+          stub_request(:post, "#{client.rest_client.uri}/push/publish").
             with do |request|
               expect(deserialize_body(request.body, protocol)['recipient']['camelCase']['secondLevelCamelCase']).to eql('val')
               expect(deserialize_body(request.body, protocol)['recipient']).to_not have_key('camel_case')
@@ -135,7 +135,7 @@
             'transportType' => 'ablyChannel',
             'channel' => channel,
             'ablyKey' => api_key,
-            'ablyUrl' => client.rest_client.endpoint.to_s
+            'ablyUrl' => client.rest_client.uri.to_s
           }
         end
         let(:notification_payload) do
diff --git a/spec/acceptance/rest/auth_spec.rb b/spec/acceptance/rest/auth_spec.rb
index ef07af9f..8e9a7866 100644
--- a/spec/acceptance/rest/auth_spec.rb
+++ b/spec/acceptance/rest/auth_spec.rb
@@ -61,7 +61,7 @@ def request_body_includes(request, protocol, key, val)
       end
 
       it 'creates a TokenRequest automatically and sends it to Ably to obtain a token', webmock: true do
-        token_request_stub = stub_request(:post, "#{client.endpoint}/keys/#{key_name}/requestToken").
+        token_request_stub = stub_request(:post, "#{client.uri}/keys/#{key_name}/requestToken").
           to_return(status: 201, body: serialize_body({}, protocol), headers: { 'Content-Type' => content_type })
         expect(auth).to receive(:create_token_request).and_call_original
         auth.request_token
@@ -90,7 +90,7 @@ def coerce_if_time_value(field_name, value, params = {})
 
           let(:token_response) { {} }
           let!(:request_token_stub) do
-            stub_request(:post, "#{client.endpoint}/keys/#{key_name}/requestToken").
+            stub_request(:post, "#{client.uri}/keys/#{key_name}/requestToken").
               with do |request|
                 request_body_includes(request, protocol, token_param, coerce_if_time_value(token_param, random, multiply: 1000))
               end.to_return(
@@ -121,7 +121,7 @@ def coerce_if_time_value(field_name, value, params = {})
 
         let(:token_response) { {} }
         let!(:request_token_stub) do
-          stub_request(:post, "#{client.endpoint}/keys/#{key_name}/requestToken").
+          stub_request(:post, "#{client.uri}/keys/#{key_name}/requestToken").
             with do |request|
               request_body_includes(request, protocol, 'mac', mac)
             end.to_return(
@@ -151,7 +151,7 @@ def coerce_if_time_value(field_name, value, params = {})
 
         let(:token_response) { {} }
         let!(:request_token_stub) do
-          stub_request(:post, "#{client.endpoint}/keys/#{key_name}/requestToken").
+          stub_request(:post, "#{client.uri}/keys/#{key_name}/requestToken").
             with do |request|
               request_body_includes(request, protocol, 'mac', mac)
             end.to_return(
@@ -293,7 +293,7 @@ def coerce_if_time_value(field_name, value, params = {})
         let(:auth_url_content_type) { 'application/json' }
 
         let!(:request_token_stub) do
-          stub_request(:post, "#{client.endpoint}/keys/#{key_name}/requestToken").
+          stub_request(:post, "#{client.uri}/keys/#{key_name}/requestToken").
             with do |request|
               request_body_includes(request, protocol, 'key_name', key_name)
             end.to_return(
@@ -1099,7 +1099,7 @@ def coerce_if_time_value(field_name, value, params = {})
               }
             }
 
-            stub_request(:post, "https://#{environment}-rest.ably.io/channels/foo/publish").
+            stub_request(:post, "https://#{client.hostname}/channels/foo/publish").
               to_return(status: 401, body: token_expired.to_json, headers: { 'Content-Type' => 'application/json' })
           end
 
@@ -1158,7 +1158,7 @@ def coerce_if_time_value(field_name, value, params = {})
           sleep 2.5
           WebMock.enable!
           WebMock.disable_net_connect!
-          stub_request(:post, "https://#{environment}-rest.ably.io/keys/#{TestApp.instance.key_name}/requestToken").
+          stub_request(:post, "https://#{client.hostname}/keys/#{TestApp.instance.key_name}/requestToken").
               to_return(status: 401, body: token_expired_response.to_json, headers: { 'Content-Type' => 'application/json' })
           expect { channel.publish 'event' }.to raise_error Ably::Exceptions::TokenExpired
           expect(auth.current_token_details).to eql(token)
diff --git a/spec/acceptance/rest/base_spec.rb b/spec/acceptance/rest/base_spec.rb
index 0e046458..12cee653 100644
--- a/spec/acceptance/rest/base_spec.rb
+++ b/spec/acceptance/rest/base_spec.rb
@@ -14,7 +14,7 @@
     let(:body_value) { [as_since_epoch(now)] }
 
     before do
-      stub_request(:get, "#{client.endpoint}/time").
+      stub_request(:get, "#{client.uri}/time").
         with(:headers => { 'Accept' => mime }).
         to_return(:status => 200, :body => request_body, :headers => { 'Content-Type' => mime })
     end
@@ -72,13 +72,13 @@
 
     describe 'failed requests' do
       context 'due to invalid Auth' do
-        it 'should raise an InvalidRequest exception with a valid error message and code' do
+        it 'should raise an UnauthorizedRequest exception with a valid error message and code' do
           invalid_client = Ably::Rest::Client.new(key: 'appid.keyuid:keysecret', environment: environment)
           expect { invalid_client.channel('test').publish('foo', 'choo') }.to raise_error do |error|
-            expect(error).to be_a(Ably::Exceptions::ResourceMissing)
-            expect(error.message).to match(/No application found/)
-            expect(error.code).to eql(40400)
-            expect(error.status).to eql(404)
+            expect(error).to be_a(Ably::Exceptions::UnauthorizedRequest)
+            expect(error.message).to match(/no application id/)
+            expect(error.code).to eql(40101)
+            expect(error.status).to eql(401)
           end
         end
       end
@@ -87,7 +87,7 @@
         let(:error_response) { '{ "error": { "statusCode": 500, "code": 50000, "message": "Internal error" } }' }
 
         before do
-          (client.fallback_hosts.map { |host| "https://#{host}" } + [client.endpoint]).each do |host|
+          (client.fallback_hosts.map { |host| "https://#{host}" } + [client.uri]).each do |host|
             stub_request(:get, "#{host}/time")
               .to_return(:status => 500, :body => error_response, :headers => { 'Content-Type' => 'application/json' })
           end
@@ -100,7 +100,7 @@
 
       describe '500 server error without a valid JSON response body', :webmock do
         before do
-          (client.fallback_hosts.map { |host| "https://#{host}" } + [client.endpoint]).each do |host|
+          (client.fallback_hosts.map { |host| "https://#{host}" } + [client.uri]).each do |host|
             stub_request(:get, "#{host}/time").
             to_return(:status => 500, :headers => { 'Content-Type' => 'application/json' })
           end
@@ -121,7 +121,7 @@
         @token_requests = 0
         @publish_attempts = 0
 
-        stub_request(:post, "#{client.endpoint}/keys/#{key_name}/requestToken").to_return do
+        stub_request(:post, "#{client.uri}/keys/#{key_name}/requestToken").to_return do
           @token_requests += 1
           {
             :body => public_send("token_#{@token_requests}").merge(expires: (Time.now.to_i + 60) * 1000).to_json,
@@ -129,7 +129,7 @@
           }
         end
 
-        stub_request(:post, "#{client.endpoint}/channels/#{channel}/publish").to_return do
+        stub_request(:post, "#{client.uri}/channels/#{channel}/publish").to_return do
           @publish_attempts += 1
           if [1, 3].include?(@publish_attempts)
             { status: 201, :body => '[]', :headers => { 'Content-Type' => 'application/json' } }
diff --git a/spec/acceptance/rest/channel_spec.rb b/spec/acceptance/rest/channel_spec.rb
index 37a4e174..36d15eab 100644
--- a/spec/acceptance/rest/channel_spec.rb
+++ b/spec/acceptance/rest/channel_spec.rb
@@ -364,12 +364,12 @@
       context 'with a non ASCII channel name' do
         let(:channel_name) { 'foo:¡€≤`☃' }
         let(:channel_name_encoded) { 'foo%3A%C2%A1%E2%82%AC%E2%89%A4%60%E2%98%83' }
-        let(:endpoint) { client.endpoint }
+        let(:uri) { client.uri }
         let(:channel) { client.channels.get(channel_name) }
 
         context 'stubbed', :webmock do
           let!(:get_stub) {
-            stub_request(:post, "#{endpoint}/channels/#{channel_name_encoded}/publish").
+            stub_request(:post, "#{uri}/channels/#{channel_name_encoded}/publish").
               to_return(:body => '{}', :headers => { 'Content-Type' => 'application/json' })
           }
 
@@ -534,8 +534,8 @@
     describe '#history option' do
       let(:channel_name) { "persisted:#{random_str(4)}" }
       let(:channel) { client.channel(channel_name) }
-      let(:endpoint) do
-        client.endpoint
+      let(:uri) do
+        client.uri
       end
       let(:default_history_options) do
           {
@@ -549,7 +549,7 @@
           let!(:history_stub) {
             query_params = default_history_options
             .merge(option => milliseconds).map { |k, v| "#{k}=#{v}" }.join('&')
-            stub_request(:get, "#{endpoint}/channels/#{URI.encode_www_form_component(channel_name)}/messages?#{query_params}").
+            stub_request(:get, "#{uri}/channels/#{URI.encode_www_form_component(channel_name)}/messages?#{query_params}").
               to_return(:body => '{}', :headers => { 'Content-Type' => 'application/json' })
           }
 
diff --git a/spec/acceptance/rest/client_spec.rb b/spec/acceptance/rest/client_spec.rb
index e1619c6e..dcbdf7ca 100644
--- a/spec/acceptance/rest/client_spec.rb
+++ b/spec/acceptance/rest/client_spec.rb
@@ -33,9 +33,9 @@ def encode64(text)
         it 'logs an entry with a help href url matching the code #TI5' do
           begin
             client.channels.get('foo').publish('test')
-            raise 'Expected Ably::Exceptions::ResourceMissing'
-          rescue Ably::Exceptions::ResourceMissing => err
-            expect err.to_s.match(%r{https://help.ably.io/error/40400})
+            raise 'Expected Ably::Exceptions::UnauthorizedRequest'
+          rescue Ably::Exceptions::UnauthorizedRequest => err
+            expect err.to_s.match(%r{https://help.ably.io/error/40101})
           end
         end
       end
@@ -139,7 +139,7 @@ def encode64(text)
           let(:client_options)      { default_options.merge(key: api_key, client_id: client_id) }
 
           let!(:get_message_history_stub) do
-            stub_request(:get, "https://#{environment}-#{Ably::Rest::Client::DOMAIN}/channels/#{channel_name}/messages?#{history_querystring}")
+            stub_request(:get, "https://#{client.hostname}/channels/#{channel_name}/messages?#{history_querystring}")
               .with(headers: { 'X-Ably-ClientId' => encode64(client_id) })
               .to_return(body: [], headers: { 'Content-Type' => 'application/json' })
           end
@@ -155,7 +155,7 @@ def encode64(text)
           let(:client_options) { default_options.merge(token: token_string) }
 
           let!(:get_message_history_stub) do
-            stub_request(:get, "#{http_protocol}://#{environment}-#{Ably::Rest::Client::DOMAIN}/channels/#{channel_name}/messages?#{history_querystring}").
+            stub_request(:get, "#{http_protocol}://#{client.hostname}/channels/#{channel_name}/messages?#{history_querystring}").
               with(headers: { 'Authorization' => "Bearer #{encode64(token_string)}" }).
               to_return(body: [], headers: { 'Content-Type' => 'application/json' })
           end
@@ -301,20 +301,20 @@ def encode64(text)
       context 'configured' do
         let(:client_options) { default_options.merge(key: api_key, environment: 'production') }
 
-        it 'should make connection attempts to a.ably-realtime.com, b.ably-realtime.com, c.ably-realtime.com, d.ably-realtime.com, e.ably-realtime.com (#RSC15a)' do
+        it 'should make connection attempts to main.a.fallback.ably-realtime.com, main.b.fallback.ably-realtime.com, main.c.fallback.ably-realtime.com, main.d.fallback.ably-realtime.com, main.e.fallback.ably-realtime.com (#RSC15a, #REC1)' do
           hosts = []
           5.times do
             hosts << client.fallback_connection.host
           end
-          expect(hosts).to match_array(%w(a.ably-realtime.com b.ably-realtime.com c.ably-realtime.com d.ably-realtime.com e.ably-realtime.com))
+          expect(hosts).to match_array(%w(main.a.fallback.ably-realtime.com main.b.fallback.ably-realtime.com main.c.fallback.ably-realtime.com main.d.fallback.ably-realtime.com main.e.fallback.ably-realtime.com))
         end
       end
 
       context 'when environment is NOT production (#RSC15b)' do
         context 'and custom fallback hosts are empty' do
-          let(:client_options) { default_options.merge(environment: 'sandbox', key: api_key, fallback_hosts: []) }
+          let(:client_options) { default_options.merge(environment: 'nonprod:sandbox', key: api_key, fallback_hosts: []) }
           let!(:default_host_request_stub) do
-            stub_request(:post, "https://#{environment}-#{Ably::Rest::Client::DOMAIN}#{path}").to_return do
+            stub_request(:post, "https://#{client.hostname}#{path}").to_return do
               raise Faraday::TimeoutError.new('timeout error message')
             end
           end
@@ -325,20 +325,20 @@ def encode64(text)
         end
 
         context 'and no custom fallback hosts are provided' do
-          let(:client_options) { default_options.merge(environment: 'sandbox', key: api_key) }
+          let(:client_options) { default_options.merge(environment: 'nonprod:sandbox', key: api_key) }
 
-          it 'should make connection attempts to sandbox-a-fallback.ably-realtime.com, sandbox-b-fallback.ably-realtime.com, sandbox-c-fallback.ably-realtime.com, sandbox-d-fallback.ably-realtime.com, sandbox-e-fallback.ably-realtime.com (#RSC15a)' do
+          it 'should make connection attempts to sandbox.a.fallback.ably-realtime-nonprod.com, sandbox.b.fallback.ably-realtime-nonprod.com, sandbox.c.fallback.ably-realtime-nonprod.com, sandbox.d.fallback.ably-realtime-nonprod.com, sandbox.e.fallback.ably-realtime-nonprod.com (#RSC15a, #REC1b3)' do
             hosts = []
             5.times do
               hosts << client.fallback_connection.host
             end
-            expect(hosts).to match_array(%w(a b c d e).map { |id| "sandbox-#{id}-fallback.ably-realtime.com" })
+            expect(hosts).to match_array(%w(a b c d e).map { |id| "sandbox.#{id}.fallback.ably-realtime-nonprod.com" })
           end
         end
       end
 
       context 'when environment is production' do
-        let(:custom_hosts)       { %w(a.ably-realtime.com b.ably-realtime.com) }
+        let(:custom_hosts)       { %w(main.a.fallback.ably-realtime.com main.b.fallback.ably-realtime.com) }
         let(:max_retry_count)    { 2 }
         let(:max_retry_duration) { 0.5 }
         let(:fallback_block)     { proc { raise Faraday::SSLError.new('ssl error message') } }
@@ -366,7 +366,7 @@ def encode64(text)
 
         context 'and connection times out' do
           let!(:default_host_request_stub) do
-            stub_request(:post, "https://#{Ably::Rest::Client::DOMAIN}#{path}").to_return do
+            stub_request(:post, "https://#{client.hostname}#{path}").to_return do
               raise Faraday::TimeoutError.new('timeout error message')
             end
           end
@@ -380,7 +380,7 @@ def encode64(text)
 
           context "and the total request time exeeds #{http_defaults.fetch(:max_retry_duration)} seconds" do
             let!(:default_host_request_stub) do
-              stub_request(:post, "https://#{Ably::Rest::Client::DOMAIN}#{path}").to_return do
+              stub_request(:post, "https://#{client.hostname}#{path}").to_return do
                 sleep max_retry_duration * 1.5
                 raise Faraday::TimeoutError.new('timeout error message')
               end
@@ -397,7 +397,7 @@ def encode64(text)
 
         context 'and connection fails' do
           let!(:default_host_request_stub) do
-            stub_request(:post, "https://#{Ably::Rest::Client::DOMAIN}#{path}").to_return do
+            stub_request(:post, "https://#{client.hostname}#{path}").to_return do
               raise Faraday::ConnectionFailed.new('connection failure error message')
             end
           end
@@ -422,7 +422,7 @@ def encode64(text)
           end
           let(:requests) { [] }
           let!(:default_host_request_stub) do
-            stub_request(:post, "https://#{Ably::Rest::Client::DOMAIN}#{path}").to_return do
+            stub_request(:post, "https://#{client.hostname}#{path}").to_return do
               requests << true
               if requests.count == 1
                 raise Faraday::ConnectionFailed.new('connection failure error message')
@@ -450,7 +450,7 @@ def encode64(text)
         context 'and basic authentication fails' do
           let(:status) { 401 }
           let!(:default_host_request_stub) do
-            stub_request(:post, "https://#{Ably::Rest::Client::DOMAIN}#{path}").to_return(
+            stub_request(:post, "https://#{client.hostname}#{path}").to_return(
               headers: { 'Content-Type' => 'application/json' },
               status: status,
               body: {
@@ -482,7 +482,7 @@ def encode64(text)
             end
           end
           let!(:default_host_request_stub) do
-            stub_request(:post, "https://#{Ably::Rest::Client::DOMAIN}#{path}").to_return(&fallback_block)
+            stub_request(:post, "https://#{client.hostname}#{path}").to_return(&fallback_block)
           end
 
           it 'attempts the fallback hosts as this is an authentication failure (#RSC15d)' do
@@ -518,7 +518,7 @@ def encode64(text)
           end
         end
         let!(:default_host_request_stub) do
-          stub_request(:post, "https://#{Ably::Rest::Client::DOMAIN}#{path}").to_return(&fallback_block)
+          stub_request(:post, "https://#{client.hostname}#{path}").to_return(&fallback_block)
         end
 
         context 'with custom fallback hosts provided' do
@@ -554,8 +554,8 @@ def encode64(text)
         end
 
         context 'using a local web-server', webmock: false do
-          let(:primary_host) { 'local-rest.ably.io' }
-          let(:fallbacks) { ['local.ably.io', 'localhost'] }
+          let(:primary_host) { 'local.realtime.ably.net' }
+          let(:fallbacks) { ['localhost.ably.net', 'localhost'] }
           let(:port) { rand(10000) + 2000 }
           let(:channel_name) { 'foo' }
           let(:request_timeout) { 3 }
@@ -838,7 +838,7 @@ def encode64(text)
 
       context 'when environment is not production and server returns a 50x error' do
         let(:env)                { 'custom-env' }
-        let(:default_fallbacks)  { %w(a b c d e).map { |id| "#{env}-#{id}-fallback.ably-realtime.com" } }
+        let(:default_fallbacks)  { %w(a b c d e).map { |id| "#{env}.#{id}.fallback.ably-realtime.com" } }
         let(:custom_hosts)       { %w(A.foo.com B.foo.com) }
         let(:max_retry_count)    { 2 }
         let(:max_retry_duration) { 0.5 }
@@ -863,7 +863,7 @@ def encode64(text)
           end
         end
         let!(:default_host_request_stub) do
-          stub_request(:post, "https://#{env}-#{Ably::Rest::Client::DOMAIN}#{path}").to_return(&fallback_block)
+          stub_request(:post, "https://#{client.hostname}#{path}").to_return(&fallback_block)
         end
 
         context 'with no fallback hosts provided (#TBC, see https://github.com/ably/wiki/issues/361)' do
@@ -1086,7 +1086,7 @@ def encode64(text)
           let(:client_options) { default_options.merge(key: api_key, agent: agent) }
 
           let!(:publish_message_stub) do
-            stub_request(:post, "#{client.endpoint}/channels/foo/publish").
+            stub_request(:post, "#{client.uri}/channels/foo/publish").
               with(headers: {
                 'X-Ably-Version' => Ably::PROTOCOL_VERSION,
                 'Ably-Agent' => agent || Ably::AGENT
@@ -1113,7 +1113,7 @@ def encode64(text)
     context '#request (#RSC19*, #TO3l9)' do
       let(:client_options) { default_options.merge(key: api_key) }
       let(:device_id) { random_str }
-      let(:endpoint) { client.endpoint }
+      let(:uri) { client.uri }
 
       context 'get' do
         it 'returns an HttpPaginatedResponse object' do
@@ -1156,7 +1156,7 @@ def encode64(text)
 
       context 'post', :webmock do
         before do
-          stub_request(:delete, "#{endpoint}/push/deviceRegistrations/#{device_id}/resetUpdateToken").
+          stub_request(:delete, "#{uri}/push/deviceRegistrations/#{device_id}/resetUpdateToken").
             to_return(status: 200, body: '{}', headers: { 'Content-Type' => 'application/json' })
         end
 
@@ -1168,14 +1168,14 @@ def encode64(text)
 
         it 'raises an exception once body size in bytes exceeded' do
           expect {
-            client.request(:post, endpoint, {}, { content: 'x' * Ably::Rest::Client::MAX_FRAME_SIZE })
+            client.request(:post, uri, {}, { content: 'x' * Ably::Rest::Client::MAX_FRAME_SIZE })
           }.to raise_error(Ably::Exceptions::MaxFrameSizeExceeded)
         end
       end
 
       context 'delete', :webmock do
         before do
-          stub_request(:delete, "#{endpoint}/push/channelSubscriptions?deviceId=#{device_id}").
+          stub_request(:delete, "#{uri}/push/channelSubscriptions?deviceId=#{device_id}").
             to_return(status: 200, body: '{}', headers: { 'Content-Type' => 'application/json' })
         end
 
@@ -1190,7 +1190,7 @@ def encode64(text)
         let(:body_params) { { 'metadata' => { 'key' => 'value' } } }
 
         before do
-          stub_request(:patch, "#{endpoint}/push/deviceRegistrations/#{device_id}")
+          stub_request(:patch, "#{uri}/push/deviceRegistrations/#{device_id}")
             .with(body: serialize_body(body_params, protocol))
             .to_return(status: 200, body: '{}', headers: { 'Content-Type' => 'application/json' })
         end
@@ -1203,7 +1203,7 @@ def encode64(text)
 
         it 'raises an exception once body size in bytes exceeded' do
           expect {
-            client.request(:patch, endpoint, {}, { content: 'x' * Ably::Rest::Client::MAX_FRAME_SIZE })
+            client.request(:patch, uri, {}, { content: 'x' * Ably::Rest::Client::MAX_FRAME_SIZE })
           }.to raise_error(Ably::Exceptions::MaxFrameSizeExceeded)
         end
       end
@@ -1219,7 +1219,7 @@ def encode64(text)
         end
 
         before do
-          stub_request(:put, "#{endpoint}/push/deviceRegistrations/#{device_id}")
+          stub_request(:put, "#{uri}/push/deviceRegistrations/#{device_id}")
             .with(body: serialize_body(body_params, protocol))
             .to_return(status: 200, body: '{}', headers: { 'Content-Type' => 'application/json' })
         end
@@ -1232,7 +1232,7 @@ def encode64(text)
 
         it 'raises an exception once body size in bytes exceeded' do
           expect {
-            client.request(:put, endpoint, {}, { content: 'x' * Ably::Rest::Client::MAX_FRAME_SIZE })
+            client.request(:put, uri, {}, { content: 'x' * Ably::Rest::Client::MAX_FRAME_SIZE })
           }.to raise_error(Ably::Exceptions::MaxFrameSizeExceeded)
         end
       end
@@ -1246,7 +1246,7 @@ def encode64(text)
 
           before do
             @request_id = nil
-            stub_request(:get, Addressable::Template.new("#{client.endpoint}/time{?request_id}")).with do |request|
+            stub_request(:get, Addressable::Template.new("#{client.uri}/time{?request_id}")).with do |request|
               @request_id = request.uri.query_values['request_id']
             end.to_return do
               raise Faraday::TimeoutError.new('timeout error message')
@@ -1268,7 +1268,7 @@ def encode64(text)
 
           context 'with mocks to inspect the params', :webmock do
             before do
-              stub_request(:post, Addressable::Template.new("#{client.endpoint}/channels/#{channel_name}/publish{?request_id}")).
+              stub_request(:post, Addressable::Template.new("#{client.uri}/channels/#{channel_name}/publish{?request_id}")).
                 with do |request|
                   @request_id = request.uri.query_values['request_id']
                 end.to_return(:status => 200, :body => [], :headers => { 'Content-Type' => 'application/json' })
@@ -1312,7 +1312,7 @@ def encode64(text)
 
           before do
             @request_id = nil
-            hosts = Ably::FALLBACK_HOSTS + ['rest.ably.io']
+            hosts = Ably::FALLBACK_HOSTS + ['main.realtime.ably.net']
             hosts.each do |host|
               stub_request(:get, Addressable::Template.new("https://#{host.downcase}/time{?request_id}")).with do |request|
                 @request_id = request.uri.query_values['request_id']
@@ -1374,7 +1374,7 @@ def encode64(text)
         let(:client_options) do
           default_options.merge(
             rest_host: 'non.existent.domain.local',
-            fallback_hosts: [[environment, Ably::Rest::Client::DOMAIN].join('-')],
+            fallback_hosts: ["sandbox.realtime.ably-nonprod.net"],
             key: api_key,
             logger: custom_logger,
             log_retries_as_info: false)
diff --git a/spec/acceptance/rest/message_spec.rb b/spec/acceptance/rest/message_spec.rb
index 49960226..5cb15307 100644
--- a/spec/acceptance/rest/message_spec.rb
+++ b/spec/acceptance/rest/message_spec.rb
@@ -220,7 +220,7 @@
       end
 
       context 'when idempotent publishing is enabled in the client library ClientOptions (#TO3n)' do
-        let(:client_options) { default_client_options.merge(idempotent_rest_publishing: true, log_level: :error, fallback_hosts: ["#{environment}-realtime.ably.io"]) }
+        let(:client_options) { default_client_options.merge(idempotent_rest_publishing: true, log_level: :error, fallback_hosts: ["#{environment.gsub("nonprod:", "")}-realtime.ably.io"]) }
 
         context 'when there is a network failure triggering an automatic retry (#RSL1k4)' do
           def mock_for_two_publish_failures
diff --git a/spec/acceptance/rest/presence_spec.rb b/spec/acceptance/rest/presence_spec.rb
index b0c61c8e..bd76f460 100644
--- a/spec/acceptance/rest/presence_spec.rb
+++ b/spec/acceptance/rest/presence_spec.rb
@@ -68,12 +68,12 @@
               limit: 100
             }
           end
-          let(:endpoint) do
-            client.endpoint
+          let(:uri) do
+            client.uri
           end
           let!(:get_stub) {
             query_params = query_options.map { |k, v| "#{k}=#{v}" }.join('&')
-            stub_request(:get, "#{endpoint}/channels/#{URI.encode_www_form_component(channel_name)}/presence?#{query_params}").
+            stub_request(:get, "#{uri}/channels/#{URI.encode_www_form_component(channel_name)}/presence?#{query_params}").
               to_return(:body => '{}', :headers => { 'Content-Type' => 'application/json' })
           }
           let(:channel_name) { random_str }
@@ -115,12 +115,12 @@
         context 'with a non ASCII channel name' do
           let(:channel_name) { 'foo:¡€≤`☃' }
           let(:channel_name_encoded) { 'foo%3A%C2%A1%E2%82%AC%E2%89%A4%60%E2%98%83' }
-          let(:endpoint) { client.endpoint }
+          let(:uri) { client.uri }
           let(:channel) { client.channels.get(channel_name) }
 
           context 'stubbed', :webmock do
             let!(:get_stub) {
-              stub_request(:get, "#{endpoint}/channels/#{channel_name_encoded}/presence?limit=100").
+              stub_request(:get, "#{uri}/channels/#{channel_name_encoded}/presence?limit=100").
                 to_return(:body => '{}', :headers => { 'Content-Type' => 'application/json' })
             }
 
@@ -197,8 +197,8 @@
         let(:presence) { client.channel(channel_name).presence }
         let(:user) { 'appid.keyuid' }
         let(:secret) { random_str(8) }
-        let(:endpoint) do
-          client.endpoint
+        let(:uri) do
+          client.uri
         end
         let(:client) do
           Ably::Rest::Client.new(key: "#{user}:#{secret}")
@@ -213,7 +213,7 @@
         context 'limit options', :webmock do
           let!(:history_stub) {
             query_params = history_options.map { |k, v| "#{k}=#{v}" }.join('&')
-            stub_request(:get, "#{endpoint}/channels/#{URI.encode_www_form_component(channel_name)}/presence/history?#{query_params}").
+            stub_request(:get, "#{uri}/channels/#{URI.encode_www_form_component(channel_name)}/presence/history?#{query_params}").
               to_return(:body => '{}', :headers => { 'Content-Type' => 'application/json' })
           }
 
@@ -253,7 +253,7 @@
               }
               let!(:history_stub) {
                 query_params = history_options.map { |k, v| "#{k}=#{v}" }.join('&')
-                stub_request(:get, "#{endpoint}/channels/#{URI.encode_www_form_component(channel_name)}/presence/history?#{query_params}").
+                stub_request(:get, "#{uri}/channels/#{URI.encode_www_form_component(channel_name)}/presence/history?#{query_params}").
                   to_return(:body => '{}', :headers => { 'Content-Type' => 'application/json' })
               }
 
@@ -322,8 +322,8 @@ def message(client_id, messages)
     describe 'decoding permutations using mocked #history', :webmock do
       let(:user) { 'appid.keyuid' }
       let(:secret) { random_str(8) }
-      let(:endpoint) do
-        client.endpoint
+      let(:uri) do
+        client.uri
       end
       let(:client) do
         Ably::Rest::Client.new(client_options.merge(key: "#{user}:#{secret}"))
@@ -357,7 +357,7 @@ def message(client_id, messages)
 
         context '#get' do
           let!(:get_stub)   {
-            stub_request(:get, "#{endpoint}/channels/#{URI.encode_www_form_component(channel_name)}/presence?limit=100").
+            stub_request(:get, "#{uri}/channels/#{URI.encode_www_form_component(channel_name)}/presence?limit=100").
               to_return(:body => serialized_encoded_message, :headers => { 'Content-Type' => content_type })
           }
 
@@ -374,7 +374,7 @@ def message(client_id, messages)
 
         context '#history' do
           let!(:history_stub)   {
-            stub_request(:get, "#{endpoint}/channels/#{URI.encode_www_form_component(channel_name)}/presence/history?direction=backwards&limit=100").
+            stub_request(:get, "#{uri}/channels/#{URI.encode_www_form_component(channel_name)}/presence/history?direction=backwards&limit=100").
               to_return(:body => serialized_encoded_message, :headers => { 'Content-Type' => content_type })
           }
 
@@ -404,7 +404,7 @@ def message(client_id, messages)
         context '#get' do
           let(:client_options) { default_options.merge(log_level: :fatal) }
           let!(:get_stub)   {
-            stub_request(:get, "#{endpoint}/channels/#{URI.encode_www_form_component(channel_name)}/presence?limit=100").
+            stub_request(:get, "#{uri}/channels/#{URI.encode_www_form_component(channel_name)}/presence?limit=100").
               to_return(:body => serialized_encoded_message_with_invalid_encoding, :headers => { 'Content-Type' => content_type })
           }
           let(:presence_message) { presence.get.items.first }
@@ -428,7 +428,7 @@ def message(client_id, messages)
         context '#history' do
           let(:client_options) { default_options.merge(log_level: :fatal) }
           let!(:history_stub)   {
-            stub_request(:get, "#{endpoint}/channels/#{URI.encode_www_form_component(channel_name)}/presence/history?direction=backwards&limit=100").
+            stub_request(:get, "#{uri}/channels/#{URI.encode_www_form_component(channel_name)}/presence/history?direction=backwards&limit=100").
               to_return(:body => serialized_encoded_message_with_invalid_encoding, :headers => { 'Content-Type' => content_type })
           }
           let(:presence_message) { presence.history.items.first }
diff --git a/spec/acceptance/rest/push_admin_spec.rb b/spec/acceptance/rest/push_admin_spec.rb
index a5e1433f..36824fa4 100644
--- a/spec/acceptance/rest/push_admin_spec.rb
+++ b/spec/acceptance/rest/push_admin_spec.rb
@@ -90,7 +90,7 @@
         end
 
         let!(:publish_stub) do
-          stub_request(:post, "#{client.endpoint}/push/publish").
+          stub_request(:post, "#{client.uri}/push/publish").
             with do |request|
               expect(deserialize_body(request.body, protocol)['recipient']['camelCase']['secondLevelCamelCase']).to eql('val')
               expect(deserialize_body(request.body, protocol)['recipient']).to_not have_key('camel_case')
@@ -119,7 +119,7 @@
             'transportType' => 'ablyChannel',
             'channel' => channel,
             'ablyKey' => api_key,
-            'ablyUrl' => client.endpoint.to_s
+            'ablyUrl' => client.uri.to_s
           }
         end
         let(:notification_payload) do
diff --git a/spec/rspec_config.rb b/spec/rspec_config.rb
index c299da4a..c498afc6 100644
--- a/spec/rspec_config.rb
+++ b/spec/rspec_config.rb
@@ -35,7 +35,7 @@
       key_name: 'app_id.key_name',
       key_secret: 'secret',
       api_key: 'app_id.key_name:secret',
-      environment: 'sandbox'
+      environment: 'nonprod:sandbox'
     ))
     WebMock.enable!
   end
diff --git a/spec/shared/client_initializer_behaviour.rb b/spec/shared/client_initializer_behaviour.rb
index 84ff2df4..0bdec092 100644
--- a/spec/shared/client_initializer_behaviour.rb
+++ b/spec/shared/client_initializer_behaviour.rb
@@ -1,14 +1,6 @@
 # encoding: utf-8
 
 shared_examples 'a client initializer' do
-  def subdomain
-    if rest?
-      'rest'
-    else
-      'realtime'
-    end
-  end
-
   def protocol
     if rest?
       'http'
@@ -154,38 +146,130 @@ def rest?
       end
     end
 
-    context 'endpoint' do
+    context 'uri' do
       before do
         allow_any_instance_of(subject.class).to receive(:auto_connect).and_return(false)
       end
 
       it 'defaults to production' do
-        expect(subject.endpoint.to_s).to eql("#{protocol}s://#{subdomain}.ably.io")
+        expect(subject.uri.to_s).to eql("#{protocol}s://main.realtime.ably.net")
+      end
+
+      context 'with endpoint option' do
+        context 'specifying a routing policy id (REC1b4)' do
+          let(:client_options) { default_options.merge(endpoint: 'test', auto_connect: false) }
+
+          it 'returns the routing policy uri' do
+            expect(subject.uri.to_s).to eql("#{protocol}s://test.realtime.ably.net")
+          end
+        end
+
+        context 'specifying a nonprod routing policy id (REC1b3)' do
+          let(:client_options) { default_options.merge(endpoint: 'nonprod:test', auto_connect: false) }
+
+          it 'returns the nonprod routing policy uri' do
+            expect(subject.uri.to_s).to eql("#{protocol}s://test.realtime.ably-nonprod.net")
+          end
+        end
+
+        context 'specifying a uri (REC1b2)' do
+          let(:client_options) { default_options.merge(endpoint: 'example.com', auto_connect: false) }
+
+          it 'returns the uri' do
+            expect(subject.uri.to_s).to eql("#{protocol}s://example.com")
+          end
+        end
+
+        context 'specifying an IPv4 address (REC1b2)' do
+          let(:client_options) { default_options.merge(endpoint: '127.0.0.1', auto_connect: false) }
+
+          it 'returns the IP address' do
+            expect(subject.uri.to_s).to eql("#{protocol}s://127.0.0.1")
+          end
+        end
+
+        context 'specifying an IPv6 address (REC1b2)' do
+          let(:client_options) { default_options.merge(endpoint: '::1', auto_connect: false) }
+
+          it 'returns the IP address' do
+            expect(subject.uri.to_s).to eql("#{protocol}s://[::1]")
+          end
+        end
+
+        context 'specifying localhost (REC1b2)' do
+          let(:client_options) { default_options.merge(endpoint: 'localhost', auto_connect: false) }
+
+          it 'returns localhost' do
+            expect(subject.uri.to_s).to eql("#{protocol}s://localhost")
+          end
+        end
       end
 
       context 'with environment option' do
-        let(:client_options) { default_options.merge(environment: 'sandbox', auto_connect: false) }
+        context 'specifying a routing policy id (REC1b4)' do
+          let(:client_options) { default_options.merge(endpoint: 'test', auto_connect: false) }
+
+          it 'returns the routing policy uri' do
+            expect(subject.uri.to_s).to eql("#{protocol}s://test.realtime.ably.net")
+          end
+        end
+
+        context 'specifying a nonprod routing policy id (REC1b3)' do
+          let(:client_options) { default_options.merge(endpoint: 'nonprod:test', auto_connect: false) }
+
+          it 'returns the nonprod routing policy uri' do
+            expect(subject.uri.to_s).to eql("#{protocol}s://test.realtime.ably-nonprod.net")
+          end
+        end
+
+        context 'specifying a uri (REC1b2)' do
+          let(:client_options) { default_options.merge(endpoint: 'example.com', auto_connect: false) }
+
+          it 'returns the uri' do
+            expect(subject.uri.to_s).to eql("#{protocol}s://example.com")
+          end
+        end
 
-        it 'uses an alternate endpoint' do
-          expect(subject.endpoint.to_s).to eql("#{protocol}s://sandbox-#{subdomain}.ably.io")
+        context 'specifying an IPv4 address (REC1b2)' do
+          let(:client_options) { default_options.merge(endpoint: '127.0.0.1', auto_connect: false) }
+
+          it 'returns the IP address' do
+            expect(subject.uri.to_s).to eql("#{protocol}s://127.0.0.1")
+          end
+        end
+
+        context 'specifying an IPv6 address (REC1b2)' do
+          let(:client_options) { default_options.merge(endpoint: '::1', auto_connect: false) }
+
+          it 'returns the IP address' do
+            expect(subject.uri.to_s).to eql("#{protocol}s://[::1]")
+          end
+        end
+
+        context 'specifying localhost (REC1b2)' do
+          let(:client_options) { default_options.merge(endpoint: 'localhost', auto_connect: false) }
+
+          it 'returns localhost' do
+            expect(subject.uri.to_s).to eql("#{protocol}s://localhost")
+          end
         end
       end
 
       context 'with rest_host option' do
         let(:client_options) { default_options.merge(rest_host: 'custom-rest.host.com', auto_connect: false) }
 
-        it 'uses an alternate endpoint for REST clients' do
+        it 'uses an alternate uri for REST clients' do
           skip 'does not apply as testing a Realtime client' unless rest?
-          expect(subject.endpoint.to_s).to eql("#{protocol}s://custom-rest.host.com")
+          expect(subject.uri.to_s).to eql("#{protocol}s://custom-rest.host.com")
         end
       end
 
       context 'with realtime_host option' do
         let(:client_options) { default_options.merge(realtime_host: 'custom-realtime.host.com', auto_connect: false) }
 
-        it 'uses an alternate endpoint for Realtime clients' do
+        it 'uses an alternate uri for Realtime clients' do
           skip 'does not apply as testing a REST client' if rest?
-          expect(subject.endpoint.to_s).to eql("#{protocol}s://custom-realtime.host.com")
+          expect(subject.uri.to_s).to eql("#{protocol}s://custom-realtime.host.com")
         end
       end
 
@@ -193,7 +277,7 @@ def rest?
         let(:client_options) { default_options.merge(port: 999, tls: false, auto_connect: false) }
 
         it 'uses the custom port for non-TLS requests' do
-          expect(subject.endpoint.to_s).to include(":999")
+          expect(subject.uri.to_s).to include(":999")
         end
       end
 
@@ -201,7 +285,7 @@ def rest?
         let(:client_options) { default_options.merge(tls_port: 666, tls: true, auto_connect: false) }
 
         it 'uses the custom port for TLS requests' do
-          expect(subject.endpoint.to_s).to include(":666")
+          expect(subject.uri.to_s).to include(":666")
         end
       end
     end
@@ -219,7 +303,7 @@ def rest?
         end
 
         it 'uses HTTP' do
-          expect(subject.endpoint.to_s).to eql("#{protocol}://#{subdomain}.ably.io")
+          expect(subject.uri.to_s).to eql("#{protocol}://main.realtime.ably.net")
         end
       end
 
@@ -266,17 +350,62 @@ def rest?
       end
     end
 
+    context 'endpoint' do
+      context 'when set without custom fallback hosts configured' do
+        let(:endpoint) { 'foo' }
+        let(:client_options) { default_options.merge(endpoint: endpoint) }
+        let(:default_fallbacks) { %w(a b c d e).map { |id| "#{endpoint}.#{id}.fallback.ably-realtime.com" } }
+
+        it 'sets the endpoint attribute' do
+          expect(subject.endpoint).to eql(endpoint)
+        end
+
+        it 'uses the default fallback hosts (#REC2c1)' do
+          expect(subject.fallback_hosts.sort).to eql(default_fallbacks)
+        end
+      end
+
+      context 'when set with custom fallback hosts configured' do
+        let(:endpoint) { 'foo' }
+        let(:custom_fallbacks) { %w(a b c).map { |id| "#{endpoint}-#{id}.foo.com" } }
+        let(:client_options) { default_options.merge(endpoint: endpoint, fallback_hosts: custom_fallbacks) }
+
+        it 'sets the endpoint attribute' do
+          expect(subject.endpoint).to eql(endpoint)
+        end
+
+        it 'uses the custom provided fallback hosts (#REC2a2)' do
+          expect(subject.fallback_hosts.sort).to eql(custom_fallbacks)
+        end
+      end
+
+      context 'when set with fallback_hosts_use_default' do
+        let(:endpoint) { 'foo' }
+        let(:custom_fallbacks) { %w(a b c).map { |id| "#{endpoint}-#{id}.foo.com" } }
+        let(:default_production_fallbacks) { %w(a b c d e).map { |id| "main.#{id}.fallback.ably-realtime.com" } }
+        let(:client_options) { default_options.merge(endpoint: endpoint, fallback_hosts_use_default: true) }
+
+        it 'sets the endpoint attribute' do
+          expect(subject.endpoint).to eql(endpoint)
+        end
+
+        it 'uses the production default fallback hosts (#REC2b)' do
+          expect(subject.fallback_hosts.sort).to eql(default_production_fallbacks)
+        end
+      end
+    end
+
     context 'environment' do
       context 'when set without custom fallback hosts configured' do
         let(:environment) { 'foo' }
         let(:client_options) { default_options.merge(environment: environment) }
-        let(:default_fallbacks) { %w(a b c d e).map { |id| "#{environment}-#{id}-fallback.ably-realtime.com" } }
+        let(:default_fallbacks) { %w(a b c d e).map { |id| "#{environment}.#{id}.fallback.ably-realtime.com" } }
 
         it 'sets the environment attribute' do
           expect(subject.environment).to eql(environment)
         end
 
-        it 'uses the default fallback hosts (#TBC, see https://github.com/ably/wiki/issues/361)' do
+        it 'uses the default fallback hosts (#REC2c1)' do
           expect(subject.fallback_hosts.sort).to eql(default_fallbacks)
         end
       end
@@ -290,7 +419,7 @@ def rest?
           expect(subject.environment).to eql(environment)
         end
 
-        it 'uses the custom provided fallback hosts (#RSC15a)' do
+        it 'uses the custom provided fallback hosts (#REC2a2)' do
           expect(subject.fallback_hosts.sort).to eql(custom_fallbacks)
         end
       end
@@ -298,14 +427,14 @@ def rest?
       context 'when set with fallback_hosts_use_default' do
         let(:environment) { 'foo' }
         let(:custom_fallbacks) { %w(a b c).map { |id| "#{environment}-#{id}.foo.com" } }
-        let(:default_production_fallbacks) { %w(a b c d e).map { |id| "#{id}.ably-realtime.com" } }
+        let(:default_production_fallbacks) { %w(a b c d e).map { |id| "main.#{id}.fallback.ably-realtime.com" } }
         let(:client_options) { default_options.merge(environment: environment, fallback_hosts_use_default: true) }
 
         it 'sets the environment attribute' do
           expect(subject.environment).to eql(environment)
         end
 
-        it 'uses the production default fallback hosts (#RTN17b)' do
+        it 'uses the production default fallback hosts (#REC2b)' do
           expect(subject.fallback_hosts.sort).to eql(default_production_fallbacks)
         end
       end
diff --git a/spec/support/test_app.rb b/spec/support/test_app.rb
index 2e7fc394..e1bf93ae 100644
--- a/spec/support/test_app.rb
+++ b/spec/support/test_app.rb
@@ -57,7 +57,7 @@ def restricted_api_key
   def delete
     return unless TestApp.instance_variable_get('@singleton__instance__')
 
-    url = "#{sandbox_client.endpoint}/apps/#{app_id}"
+    url = "#{sandbox_client.uri}/apps/#{app_id}"
 
     basic_auth = Base64.urlsafe_encode64(api_key).chomp
     headers    = { "Authorization" => "Basic #{basic_auth}" }
@@ -66,11 +66,11 @@ def delete
   end
 
   def environment
-    ENV['ABLY_ENV'] || 'sandbox'
+    ENV['ABLY_ENV'] || 'nonprod:sandbox'
   end
 
   def create_test_app
-    url = "#{sandbox_client.endpoint}/apps"
+    url = "#{sandbox_client.uri}/apps"
 
     headers = {
       'Accept'       => 'application/json',
@@ -86,7 +86,7 @@ def create_test_app
   end
 
   def host
-    sandbox_client.endpoint.host
+    sandbox_client.uri.host
   end
 
   def realtime_host
@@ -94,12 +94,13 @@ def realtime_host
   end
 
   def create_test_stats(stats)
-    client = Ably::Rest::Client.new(key: api_key, environment: environment)
+    client = Ably::Rest::Client.new(key: api_key, endpoint: environment)
     response = client.post('/stats', stats)
     raise "Could not create stats fixtures.  Ably responded with status #{response.status}\n#{response.body}" unless (200..299).include?(response.status)
   end
 
   private
+
   def sandbox_client
     @sandbox_client ||= Ably::Rest::Client.new(key: 'app.key:secret', tls: true, environment: environment)
   end
diff --git a/spec/unit/models/http_paginated_result_spec.rb b/spec/unit/models/http_paginated_result_spec.rb
index 1c4ac199..cb8ef1bc 100644
--- a/spec/unit/models/http_paginated_result_spec.rb
+++ b/spec/unit/models/http_paginated_result_spec.rb
@@ -23,7 +23,7 @@
       status: status
     })
   end
-  let(:base_url) { 'http://rest.ably.io/channels/channel_name' }
+  let(:base_url) { 'http://main.realtime.ably.net/channels/channel_name' }
   let(:full_url) { "#{base_url}/whatever?param=exists" }
   let(:paginated_result_options) { Hash.new }
   let(:first_paged_request) { paginated_result_class.new(http_response, full_url, client, paginated_result_options) }
@@ -193,7 +193,7 @@
   end
 
   context 'with paged http response' do
-    let(:base_url) { 'http://rest.ably.io/channels/channel_name' }
+    let(:base_url) { 'http://main.realtime.ably.net/channels/channel_name' }
     let(:full_url) { "#{base_url}/messages" }
     let(:headers) do
       {
diff --git a/spec/unit/realtime/client_spec.rb b/spec/unit/realtime/client_spec.rb
index 48eb81c7..afb70841 100644
--- a/spec/unit/realtime/client_spec.rb
+++ b/spec/unit/realtime/client_spec.rb
@@ -13,7 +13,7 @@
     let(:client_options) { { key: 'appid.keyuid:keysecret', auto_connect: false } }
 
     it 'passes on the options to the initializer' do
-      rest_client = instance_double('Ably::Rest::Client', auth: instance_double('Ably::Auth'), options: client_options, environment: 'production', use_tls?: true, custom_tls_port: nil)
+      rest_client = instance_double('Ably::Rest::Client', auth: instance_double('Ably::Auth'), options: client_options, environment: 'production', use_tls?: true, custom_tls_port: nil, endpoint: 'main')
       expect(Ably::Rest::Client).to receive(:new).with(hash_including(client_options)).and_return(rest_client)
       realtime_client
     end
@@ -25,6 +25,11 @@
           realtime_client.public_send attribute
         end
       end
+
+      specify "#endpoint (#REC1)" do
+        expect(realtime_client.rest_client).to receive(:endpoint)
+        realtime_client.endpoint
+      end
     end
   end
 
diff --git a/spec/unit/realtime/connection_spec.rb b/spec/unit/realtime/connection_spec.rb
index 9c227af0..60aa9599 100644
--- a/spec/unit/realtime/connection_spec.rb
+++ b/spec/unit/realtime/connection_spec.rb
@@ -2,7 +2,8 @@
 require 'shared/protocol_msgbus_behaviour'
 
 describe Ably::Realtime::Connection do
-  let(:client) { instance_double('Ably::Realtime::Client', logger: double('logger').as_null_object, recover: nil, endpoint: double('endpoint', host: 'realtime.ably.io')) }
+  # Using main.realtime.ably.net as the default hostname (#REC1)
+  let(:client) { instance_double('Ably::Realtime::Client', logger: double('logger').as_null_object, recover: nil, uri: double('uri', host: 'main.realtime.ably.net')) }
 
   subject do
     Ably::Realtime::Connection.new(client, {}).tap do |connection|