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, they do 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 client-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 requiring a full IPv4 stack, making it an efficient solution for networks transitioning to IPv6 while maintaining compatibility with legacy IPv4 systems. To support 464XLAT, Windows provides specific functionality for CLAT, though with some limitations.
CLAT for Windows
Windows currently provides CLAT support only for cellular network interfaces. CLAT is not available for Wi-Fi or Ethernet interfaces today. However, Microsoft has publicly announced plans to extend CLAT support in Windows for these non-cellular network interfaces soon.
IPv6 Mostly
IPv6 Mostly, defined in RFC 8925, refers to a network configuration where IPv6 is the primary protocol for communication, but IPv4 is still supported for specific use cases. Devices in these networks prefer IPv6 for most operations, leveraging its larger address space and modern features, while maintaining limited IPv4 compatibility. IPv6 Mostly networks ease the transition from IPv4 to IPv6, balancing modern protocol adoption with support for older applications. They optimize resource usage and prepare networks for a future where IPv6 dominates, with tools like 464XLAT providing seamless IPv4 access when necessary.
DHCP Option 108
DHCP Option 108 is a specific configuration in DHCP that enables IPv6-only networks to signal clients to disable IPv4. When a client receives this option, it deactivates its IPv4 stack, relying solely on IPv6 for communication. Turning off IPv4 when it isn’t needed helps streamline network operations in IPv6-focused environments.
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 represents the duration for which a client should prefer IPv6-only mode. 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 indicates support for DHCP option 108 in its DHCP Request, the DHCP server will include it in the DHCP Offer, as shown here.
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]
Microsoft Plans to Extend CLAT Support in Windows 11
RFC 6877 – 464XLAT: Combination of Stateful and Stateless Translation
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