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).
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.
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