An IPv6-mostly network allows IPv6-only, dual-stack, and IPv4-only devices to coexist on the same network segment. Devices capable of operating without native IPv4 connectivity use IPv6, while devices that still require IPv4 continue to receive an IPv4 address. This approach allows organizations to move toward IPv6-only operation incrementally while maintaining compatibility with existing devices and applications.
Why IPv6-Mostly?
Many organizations deploy IPv6 alongside IPv4, a configuration known as dual stack. However, assigning both protocols to every device keeps demand for IPv4 addresses and requires administrators to manage both protocols across the network. IPv6-mostly provides a practical next step by letting capable devices operate without native IPv4 connectivity while retaining IPv4 for devices that need it.
DHCP Option 108
DHCPv4 option 108, called IPv6-Only Preferred, provides a way for the network and client to agree that native IPv4 configuration is unnecessary. A compatible client requests this option, and the DHCP server includes it in its response when the network supports IPv6-only operation. The client then forgoes obtaining an IPv4 address for the specified period. Devices that do not support this option continue obtaining IPv4 addresses normally.
Access to IPv4 Resources
Operating without native IPv4 connectivity does not eliminate the need to access IPv4 resources. NAT64 provides translation between IPv6 clients and IPv4 destinations, while DNS64 can synthesize IPv6 DNS records for IPv4-only resources. Applications that generate IPv4 traffic require a customer-side translator (CLAT) working with NAT64, an architecture known as 464XLAT. On Windows, Windows CLAT (WinCLAT) provides this functionality.
Deployment Considerations
IPv6-mostly requires working IPv6 connectivity, appropriate translation services, and validation of device and application compatibility. Enabling DHCP option 108 alone does not provide these capabilities. Administrators should test with a limited group of devices before expanding deployment. The IETF discusses this approach in IPv6-mostly Networks: Deployment and Operations Considerations, which is currently an Internet-Draft and remains a work in progress.
Summary
IPv6-mostly networks allow capable devices to operate using IPv6 while retaining native IPv4 connectivity for devices that need it. DHCP option 108 and translation technologies such as NAT64 and 464XLAT support this gradual transition toward IPv6-only networking.
