"The basic concept of NAT is to provide a mechanism to allow for IP communication between end stations when there is a reason that one end wouldn't normally be able to reach the other. For example if a host that is on a network that does not have Internet routable addresses needs to access a host that does have a routable IP address. Sound confusing? Well, it really isn't. The bottom line (and the aspect of NAT that I will focus on) is that the Internet is composed of many networks and the way that data traverses these networks is by following routes to particular addresses, the IP address of the stations that make up the Internet. Now, the tricky part is that while many networks are connected to the Internet, not all of them have addresses that are legal registered to be routed on the Internet. "