While enterprise adoption of IPv6 has been slow, it is still moving forward. For example, the U.S. federal government has mandated [M-21-07 – PDF] the transition to IPv6 to modernize its networks and enhance security, scalability, and interoperability. During the migration to IPv6, most systems will be configured with both IPv4 and IPv6, a configuration referred to as dual stack. Ultimately, the goal is the elimination of IPv4 entirely and the use of IPv6 exclusively. However, IPv6-only presents some unique challenges.
Access to IPv4
Although an organization can successfully migrate to IPv6-only networks internally, it does not control networks outside its boundaries. In some cases, a host on an IPv6-only network may need to communicate with an IPv4 resource. Administrators must deploy an IPv6 transition technology to support this scenario.
464XLAT
464XLAT, defined in RFC 6877, is a network architecture that facilitates the transition from IPv4 to IPv6 by enabling IPv4 traffic to operate over an IPv6-only network. It combines two translation mechanisms: a customer-side translator (CLAT) on the user device, which converts IPv4 packets to IPv6, and a provider-side translator (PLAT) at the network edge, which converts the IPv6 packets back to IPv4 to communicate with IPv4-only internet services. This dual-translation approach allows devices in an IPv6-only environment to access both IPv6 and IPv4 resources without native IPv4 connectivity, making it an efficient solution for networks transitioning to IPv6 while maintaining compatibility with legacy IPv4 systems. On Windows, Windows CLAT (WinCLAT) provides the CLAT component.
CLAT for Windows
Windows has long included CLAT support for cellular network interfaces. Windows CLAT (WinCLAT) extends this support to Wi-Fi and Ethernet interfaces and is generally available on Windows 11 version 25H2, starting with build 26200.9550. WinCLAT is disabled by default on non-cellular interfaces and must be permitted before it can operate. Windows also honors DHCP option 108 only while WinCLAT is permitted. WinCLAT is not available on Windows Server. For configuration details, see Windows CLAT (WinCLAT) Overview and Configuration.
IPv6-Mostly
An IPv6-mostly network, as defined in RFC 8925, provides NAT64 alongside native IPv4 and allows IPv6-only, dual-stack, and IPv4-only devices to coexist on the same network segment. Devices that can operate without IPv4 forgo an IPv4 address, while devices that still require IPv4 continue to receive one. This approach reduces IPv4 address consumption and allows organizations to move toward IPv6-only incrementally, with 464XLAT providing IPv4 access for applications that still need it.
DHCP Option 108
DHCPv4 option 108, called IPv6-Only Preferred, allows an IPv6-mostly network to signal to capable clients that they can operate without a native IPv4 address. Only clients that can operate without IPv4, usually because they include a CLAT or because all of their applications work over IPv6, request the option. When the DHCP scope is configured for IPv6-mostly operation, the server includes option 108 in its response, along with a wait time. The client then stops DHCPv4 configuration for that period, or until it reconnects to a network, and relies on IPv6 and NAT64 for connectivity. Disabling the IPv4 stack entirely during that time is optional. Clients that do not request the option obtain an IPv4 address as usual.
Option 108 and Windows Server DHCP
Commercial DHCP appliances like Infoblox and many open source DHCP platforms natively support DHCP option 108. However, no supported version of Windows Server, including the latest release (Windows Server 2025), supports DHCP option 108 natively. To enable DHCP option 108 on Windows DHCP servers, administrators can create a custom predefined option.
Custom Predefined Option
To create a custom predefined option for DHCP option 108 on a Windows DHCP server, open the DHCP management console (dhcpmgmt.msc) and perform the following steps.
- Right-click IPv4 and choose Set Predefined Options.
- Click Add.
- Enter IPv6 Only Preferred in the Name field.
- Select Long from the Data type drop-down list.
- Enter 108 in the Code field.
- Click Ok.
Assigning DHCP Option 108
Once complete, perform the following steps to assign DHCP option 108 to a DHCP scope.
- Select an IPv4 DHCP scope.
- Right-click Scope Options and choose Configure Options.
- Select 108 IPv6 Only Preferred from the Available Options list.
- Enter a value in seconds, in hexadecimal format. This value is the period during which a client stops DHCPv4 configuration and operates without native IPv4. RFC 8925 sets a minimum of 300 seconds (clients treat lower values as 300) and a default of 1800 seconds. For example, 86,400 seconds (1 day) is 0x15180.
- Click Ok.
PowerShell
Custom predefined options can also be configured using PowerShell.
Custom Predefined Option
To create a custom predefined option for DHCP option 108, open an elevated PowerShell command on a Windows DHCP server and run the following command.
Add-DhcpServerv4OptionDefinition -Name 'IPv6 Only Preferred' -OptionId 108 -Type DWORD -PassThru
Assigning DHCP Option 108
To assign the custom predefined DHCP option 108 to a DHCP scope, run the following PowerShell command.
Set-DhcpServerv4OptionValue -ScopeId 172.16.5.0 -OptionId 108 -Value 0x15180 -PassThru
DHCP Offer
Once configured, if the client requests DHCP option 108 in its DHCP Discover, the DHCP server includes it in the DHCP Offer, as shown here. Windows DHCP Server does not send option 108 to clients that do not request it, as required by RFC 8925.
Learn More
If you are interested in learning more about IPv6-mostly and DHCP option 108, be sure to listen to the following episodes of the IPv6 Buzz Podcast.
- Episode 110 – The Peculiar Power of DHCP Option 108
- Episode 158 – IPv6 Mostly: A Strategy to Balance Legacy and Modern Networking Needs
- Episode 117 – Microsoft’s OS Evolution with IPv6
Summary
As organizations continue their transition toward IPv6, DHCP option 108 provides administrators with a simple and effective way to reduce reliance on legacy IPv4 by signaling clients to prefer IPv6-only operation if they can support it. While Windows Server does not natively support this option, creating a custom predefined setting ensures administrators can take advantage of this important feature.
Additional Information
M-21-07 – Completing the Transition to IPv6 for U.S. Federal Government Agencies [PDF]
Windows CLAT (WinCLAT) Overview and Configuration
How Windows CLAT (WinCLAT) Discovers the NAT64 Prefix
When Windows CLAT (WinCLAT) Is Used on IPv6-Mostly and IPv6-Only Networks
What is 464XLAT? CLAT and PLAT Explained
What is an IPv6-Mostly Network?
464XLAT: Combination of Stateful and Stateless Translation (RFC 6877)
IPv6-Only Preferred Option for DHCPv4 (RFC 8925)
