Network address translator

A network address translator (NAT) is a device, or often a software function, which maps Internet Protocol addresses between two different address spaces, most often, in an Internet Protocol version 4 context, the nonroutable "private" address space and the "public" space in which Internet Service Providers communicate. NATs are not ideal solutions for many of their uses, and one of the design goals for Internet Protocol version 6 was to do away with the need for a NAT function. Deployment experience suggests there will still be needs for some NAT-like functions in IPv6, but it is realistic to assume that they will be far less common in an all-IPv6 world.

Introduced in 1994, a NAT is a controlled violation of the "end-to-end" architectural principle of the Internet, which assumes addresses are unique from end host to end host. While NAT functions have been extremely useful, they also have created a variety of operational challenges, and one of the goals in developing Internet Protocol version 6 was to do away with the need for NAT service.