Relationship between NAT and PAT
PAT is closely related to NAT.
In NAT, generally only the IP addresses are modified: There is a 1:1 correspondence between publicly exposed IP addresses and privately held IP addresses. In PAT, both the sender’s private IP and port number are modified; the PAT device chooses the port numbers that will be seen by hosts on the public network.
In NAT, incoming packets are routed to their destination IP address on the private network by reference to the incoming source IP address given by the host on the public network. In PAT, there is generally only one publicly exposed IP address and incoming packets from the public network are routed to their destinations on the private network by reference to a table held within the PAT device that keeps track of public and private port pairs. This is often called connection tracking.
Some devices that offer NAT, such as broadband routers, actually offer PAT. For this reason, there is considerable confusion between the terms. The common use of NAT to include PAT devices suggests that PAT should be considered a type of NAT rather than a distinct technology.