If you’re a host running on an IPv6-only network, you might want to detect the IPv6 prefix used for NAT64 (for example, to transform IPv4 literals a clueless idiot embedded into a URL into IPv6 addresses).

Apple has a wonderful developer-focused page describing NAT64 and DNS64, including the way they synthesize IPv6 addresses from IPv4 literals. You (RFC 6919) MUST read it.

There’s a standard way to do that: do a AAAA DNS lookup for ipv4only.arpa, which always maps into 192.0.0.170/171, and extract the NAT64 prefix from the response.

For more details, read RFC 7050 (Discovery of the IPv6 Prefix Used for IPv6 Address Synthesis).

Hat tip to Gert Doering who pointed out ipv4only.arpa on one of the IPv6 mailing lists.

More details?

Sure. You’ll find more about NAT64 in IPv6 Transition Mechanisms webinar, and a wealth of IPv6-related information on the ipSpace.net IPv6 page.