NAT64 with DNS64
The technique that lets an IPv6-only client reach servers that only have IPv4. Necessary for a pure IPv6 network to be usable in a world that hasn't migrated.
It takes two parts. DNS64 intercepts lookups whose answer has no AAAA record and synthesizes an IPv6 address by embedding the IPv4 address in a prefix — often the well-known 64:ff9b::/96. The client believes it's speaking IPv6 all the way, but the traffic reaches a NAT64 gateway that translates to IPv4 and back. Several large mobile operators run their networks this way. What breaks is software that uses IPv4 addresses directly instead of names, since it never goes through DNS. The fix is called 464XLAT and puts a small translation layer in the client.