NAT64 is a mechanism for translating IPv6 packets to IPv4 packets and vice-versa. DNS64 is a mechanism for synthesizing AAAA records from A records. These two mechanisms together enable client-server communication between an IPv6-only client and an IPv4-only server, without requiring any changes to either the IPv6 or the IPv4 node, for the class of applications that work through NATs. They also enable peer-to-peer communication between an IPv4 and an IPv6 node, where the communication can be initiated by either end using existing, NAT-traversing, peer-to-peer communication techniques. This document specifies NAT64, and gives suggestions on how they should be deployed.