diff --git a/src/control/server/engine/config.go b/src/control/server/engine/config.go
index e49214ca111..82f122b11c5 100644
--- a/src/control/server/engine/config.go
+++ b/src/control/server/engine/config.go
@@ -48,8 +48,18 @@ type FabricConfig struct {
 	NumSecondaryEndpoints []int  `yaml:"secondary_provider_endpoints,omitempty" cmdLongFlag:"--nr_sec_ctx,nonzero" cmdShortFlag:"-S,nonzero"`
 	DisableSRX            bool   `yaml:"disable_srx,omitempty" cmdEnv:"FI_OFI_RXM_USE_SRX,invertBool,intBool"`
 	AuthKey               string `yaml:"fabric_auth_key,omitempty" cmdEnv:"D_PROVIDER_AUTH_KEY"`
+	// AddrFormat selects the preferred IP address family for fabric init. It is
+	// forwarded verbatim to CaRT/Mercury via the D_ADDR_FORMAT environment
+	// variable. An empty value leaves the historical (IPv4-preferring) default
+	// untouched. For multi-provider configurations it accepts a comma-separated
+	// list, one entry per provider, matching the provider ordering.
+	AddrFormat string `yaml:"addr_format,omitempty" cmdEnv:"D_ADDR_FORMAT"`
 }
 
+// FabricAddrFormats enumerates the address-family hints accepted by addr_format,
+// mirroring the values parsed by CaRT's crt_str_to_addr_format().
+var FabricAddrFormats = []string{"unspec", "ipv4", "ipv6", "native"}
+
 // GetPrimaryProvider parses the primary provider from the Provider string.
 func (fc *FabricConfig) GetPrimaryProvider() (string, error) {
 	providers, err := fc.GetProviders()
@@ -120,6 +130,17 @@ func (fc *FabricConfig) GetInterfaces() ([]string, error) {
 	return interfaces, nil
 }
 
+// GetAddrFormats parses the AddrFormat string into zero or more per-provider
+// address-family hints, matched one-to-one with the providers. Returns an empty
+// slice when addr_format is unset, which preserves the default behavior.
+func (fc *FabricConfig) GetAddrFormats() []string {
+	if fc == nil {
+		return nil
+	}
+
+	return splitMultiProviderStr(fc.AddrFormat)
+}
+
 // GetInterfacePorts parses the InterfacePort string to one or more ports.
 func (fc *FabricConfig) GetInterfacePorts() ([]int, error) {
 	if fc == nil {
@@ -157,6 +178,9 @@ func (fc *FabricConfig) Update(other FabricConfig) {
 	if fc.AuthKey == "" {
 		fc.AuthKey = other.AuthKey
 	}
+	if fc.AddrFormat == "" {
+		fc.AddrFormat = other.AddrFormat
+	}
 	if len(fc.NumSecondaryEndpoints) == 0 {
 		fc.setNumSecondaryEndpoints(other.NumSecondaryEndpoints)
 	}
@@ -200,6 +224,10 @@ func (fc *FabricConfig) Validate() error {
 		return errors.Errorf("provider, fabric_iface and fabric_iface_port must include the same number of items delimited by %q", MultiProviderSeparator)
 	}
 
+	if err := fc.validateAddrFormat(numProv); err != nil {
+		return err
+	}
+
 	numSecProv := numProv - numPrimaryProviders
 	if numSecProv > 0 {
 		if len(fc.NumSecondaryEndpoints) != 0 && len(fc.NumSecondaryEndpoints) != numSecProv {
@@ -216,6 +244,31 @@ func (fc *FabricConfig) Validate() error {
 	return nil
 }
 
+// validateAddrFormat ensures every comma-separated addr_format entry is a
+// recognized address-family hint and that the count matches the number of
+// providers (one hint per provider, as with fabric_iface). An empty
+// addr_format is valid and preserves the default behavior.
+func (fc *FabricConfig) validateAddrFormat(numProv int) error {
+	if fc.AddrFormat == "" {
+		return nil
+	}
+
+	formats := splitMultiProviderStr(fc.AddrFormat)
+	for _, f := range formats {
+		if !common.Includes(FabricAddrFormats, f) {
+			return errors.Errorf("invalid addr_format %q, must be one of %v",
+				f, FabricAddrFormats)
+		}
+	}
+
+	if len(formats) != numProv {
+		return errors.Errorf("addr_format must include one value per provider, delimited by %q",
+			MultiProviderSeparator)
+	}
+
+	return nil
+}
+
 // cleanEnvVars scrubs the supplied slice of environment
 // variables by removing all variables not included in the
 // allow list.
@@ -711,6 +764,14 @@ func (c *Config) WithFabricAuthKey(key string) *Config {
 	return c
 }
 
+// WithFabricAddrFormat sets the preferred IP address family for fabric init
+// (forwarded to CaRT/Mercury as D_ADDR_FORMAT). Accepted values are listed in
+// FabricAddrFormats; an empty value preserves the default behavior.
+func (c *Config) WithFabricAddrFormat(format string) *Config {
+	c.Fabric.AddrFormat = format
+	return c
+}
+
 // WithSrxDisabled disables or enables SRX.
 func (c *Config) WithSrxDisabled(disable bool) *Config {
 	c.Fabric.DisableSRX = disable
diff --git a/src/control/server/engine/config_test.go b/src/control/server/engine/config_test.go
index 1c771cfe8b5..c92139f07ad 100644
--- a/src/control/server/engine/config_test.go
+++ b/src/control/server/engine/config_test.go
@@ -693,6 +693,7 @@ func TestConfig_ToCmdVals(t *testing.T) {
 		WithFabricProvider(provider).
 		WithFabricInterface(interfaceName).
 		WithFabricInterfacePort(interfacePort).
+		WithFabricAddrFormat("ipv6").
 		WithPinnedNumaNode(pinnedNumaNode).
 		WithBypassHealthChk(&bypass).
 		WithModules(modules).
@@ -727,6 +728,7 @@ func TestConfig_ToCmdVals(t *testing.T) {
 		"D_INTERFACE=" + interfaceName,
 		"D_PORT=" + strconv.Itoa(interfacePort),
 		"D_PROVIDER=" + provider,
+		"D_ADDR_FORMAT=ipv6",
 		"D_LOG_FILE=" + logFile,
 		"D_LOG_MASK=" + logMask,
 		"CRT_TIMEOUT=" + strconv.FormatUint(uint64(crtTimeout), 10),
@@ -899,6 +901,47 @@ func TestFabricConfig_GetInterfaces(t *testing.T) {
 	}
 }
 
+func TestFabricConfig_GetAddrFormats(t *testing.T) {
+	for name, tc := range map[string]struct {
+		cfg            *FabricConfig
+		expAddrFormats []string
+	}{
+		"nil": {
+			expAddrFormats: nil,
+		},
+		"unset": {
+			cfg:            &FabricConfig{},
+			expAddrFormats: []string{},
+		},
+		"single": {
+			cfg: &FabricConfig{
+				AddrFormat: "ipv6",
+			},
+			expAddrFormats: []string{"ipv6"},
+		},
+		"multi": {
+			cfg: &FabricConfig{
+				AddrFormat: multiProviderString("ipv6", "ipv4"),
+			},
+			expAddrFormats: []string{"ipv6", "ipv4"},
+		},
+		"excessive whitespace": {
+			cfg: &FabricConfig{
+				AddrFormat: multiProviderString(" ipv6  ", "", "ipv4 "),
+			},
+			expAddrFormats: []string{"ipv6", "ipv4"},
+		},
+	} {
+		t.Run(name, func(t *testing.T) {
+			addrFormats := tc.cfg.GetAddrFormats()
+
+			if diff := cmp.Diff(tc.expAddrFormats, addrFormats); diff != "" {
+				t.Fatalf("(-want, +got):\n%s", diff)
+			}
+		})
+	}
+}
+
 func TestFabricConfig_GetPrimaryInterface(t *testing.T) {
 	for name, tc := range map[string]struct {
 		cfg          *FabricConfig
diff --git a/src/control/server/server.go b/src/control/server/server.go
index 69fa0703a4d..fafb846ebca 100644
--- a/src/control/server/server.go
+++ b/src/control/server/server.go
@@ -448,15 +448,31 @@ func (srv *server) setupGrpc() error {
 		return err
 	}
 
+	// Address family is a property of this system's fabric, so it must travel
+	// to attaching clients alongside the provider. Surface the operator's
+	// addr_format choice as a D_ADDR_FORMAT entry in the per-provider client
+	// hint env, which the client applies before crt_init (see
+	// dc_mgmt_net_cfg_init). A client/agent attached to several systems gets
+	// each system's family from that system's own hint. Empty addr_format
+	// leaves the hint untouched, preserving the historical default.
+	addrFormats := srv.cfg.Fabric.GetAddrFormats()
+
 	clientNetHints := make([]*mgmtpb.ClientNetHint, 0, len(providers))
 	for i, p := range providers {
+		envVars := srv.cfg.ClientEnvVars
+		if i < len(addrFormats) && addrFormats[i] != "" {
+			// Copy rather than append in place to avoid aliasing the shared
+			// ClientEnvVars slice across providers.
+			envVars = append(append([]string{}, envVars...), "D_ADDR_FORMAT="+addrFormats[i])
+		}
+
 		clientNetHints = append(clientNetHints, &mgmtpb.ClientNetHint{
 			Provider:    p,
 			CrtTimeout:  srv.cfg.Fabric.CrtTimeout,
 			NetDevClass: uint32(srv.netDevClass[i]),
 			SrvSrxSet:   srxSetting,
 			ProviderIdx: uint32(i),
-			EnvVars:     srv.cfg.ClientEnvVars,
+			EnvVars:     envVars,
 		})
 	}
 	srv.mgmtSvc.clientNetworkHint = clientNetHints
diff --git a/utils/config/daos_server.yml b/utils/config/daos_server.yml
index a16693c3564..0bbd8d610c3 100644
--- a/utils/config/daos_server.yml
+++ b/utils/config/daos_server.yml
@@ -136,6 +136,24 @@
 #fabric_auth_key: foo:bar
 #
 #
+## CART: Fabric address family
+## Selects the preferred IP address family for fabric initialization,
+## forwarded to CaRT/Mercury as D_ADDR_FORMAT. Leave unset to keep the
+## historical default (the provider's own preference; IPv4 for verbs/RoCE).
+## Set to "ipv6" for IPv6-only fabric NIC deployments, where the default
+## IPv4 preference would otherwise hide the only usable interfaces.
+## Accepted values: unspec (default), ipv4, ipv6, native. For multi-provider
+## configurations, supply a comma-separated list, one entry per provider.
+##
+## This value is also advertised to attaching clients (as D_ADDR_FORMAT in the
+## GetAttachInfo network hint), so clients select the same address family as
+## this system's engines without separate client configuration. A client or
+## agent attached to several systems picks up each system's family from that
+## system's hint.
+#
+#addr_format: ipv6
+#
+#
 ## Core Dump Filter
 ## Optional filter to control which mappings are written to the core
 ## dump in the event of a crash. See the following URL for more detail: