IPv6-to-IPv6 Network Prefix Translation
IPv6-to-IPv6 Network Prefix Translation is a specification for IPv6 to achieve address-independence at the network edge, similar to network address translation in Internet Protocol version 4. It has fewer architectural problems than IPv4 NAT; for example, it is stateless, uses a 1:1 address mapping and preserves the reachability attributed to the end-to-end principle. However, because the new address is chosen in a way that leaves the checksum unchanged the interface identifier bits change and this may break applications that embed data in them. Additionally, split-horizon DNS may be required for use in a business environment.
NAT66
NAT66 was the name used in earlier drafts of the standard. There were some initial proposals to rename it, and a few years later the name NPTv6 was chosen. One of the early versions defined two modes of operation within NAT66: a Two-Way Algorithmic mapping and a Topology Hiding Option, the latter of which used a non-reversible address mapping that would've required additional state in the translator, either in the form of a dynamic table or a statically defined set of address mappings. It was soon removed, leaving the two-way mapping as the only mode of operation and making the specification fully stateless.Current usage of the term is unclear: sometimes it's still employed as a synonym for NPTv6 but often it refers to a generic implementation of stateful NAT on IPv6.