TCP/IP Guide

IP NAT Static and Dynamic Address Mappings

NAT allows you to connect a private (inside) network to a public (outside) network such as the Internet by using an address translation algorithm implemented in a router that connects the two. Each time a NAT router encounters an IP datagram that crosses the boundary between the two networks, it must translate addresses as appropriate. But how does it know what to translate and what to use for the translated address?

The NAT software in the router must maintain a translation table to tell it how to operate. The translation table contains information that maps the inside local addresses of internal devices (their regular addresses) to inside global address representations (the special public addresses used ...