TCP/IP Guide

IP NAT Port-Based (Overloaded) Operation

Both traditional NAT and bidirectional NAT work by swapping inside network and outside network addresses as needed in order to allow a private network to access a public one. For each transaction, there is a one-to-one mapping between the inside local address of a device on the private network and the inside global address that represents it on the public network. We can use dynamic address assignment to allow a large number of private hosts to share a small number of registered public addresses.

However, there is a potential snag here. Consider the earlier NAT example, where 250 hosts share 20 inside global (public) addresses. If 20 hosts already have transactions in progress, what happens when the 21st ...