Learn about the mechanism of NAT (Network Address Translation) (Part 1)
nat allows one (or more) local ip addresses to be mapped to one (or more) external ip addresses. to understand more about nat as well as nat's operation mechanism, refer to theLearn about NAT (Last part)
the biggest benefit of nat is that network administration (network administation) is clear. for example, you can move your web server or ftp server to another host computer