By Brian Scheibe
Network Address Translation (NAT) and Port Address Translation (PAT) both map IP addresses on an internal network to IP addresses on an external network. Which method of address translation you use depends on the types of networks that you are translating and the number of available IP addresses that you have.
If you are connecting a site in the 10.10.10.0 network to a site in the 10.10.20.0 network, you could use NAT to translate 10.10.10.0 IP addresses to available 10.10.20.0 IP addresses so that hosts on the 10.10.10.0 network can access data and use network resources on the 10.10.20.0 network. However, for this scenario to work, you must have an address pool that contains enough available IP addresses on the 10.10.20.0 network to accommodate every host on the 10.10.10.0 network, because NAT requires a one-to-one relationship when translating IP addresses.
PAT attempts to use the original source port number of the internal host to form a unique, registered IP address and port number combination. For example, two hosts that have been assigned the IP addresses 10.10.10.100 and 10.10.10.101, respectively, could send traffic to and receive traffic from the Internet by using the single public IP address 126.96.36.199. If that port number is already allocated, PAT searches for an available alternate source port number. Therefore, the host at IP address 10.10.10.100 could access the Internet by using the public IP address and source port combination of 188.8.131.52:10000. Meanwhile, the host at IP address 10.10.10.101 could access the Internet by using the IP address and source port combination of 184.108.40.206:10001.
If you are connecting a site in the 10.10.10.0 network to the Internet, you must translate host IPs on that network to a registered IP address that is routable over the Internet. In order to use traditional NAT in this scenario, you would need to purchase a registered IP address for each host on your internal network. Alternatively, you could use PAT to translate all the IP addresses on the internal network to a single, shared IP address that connects to the Internet. PAT, which is also known as NAT overloading, uses 16-bit source port numbers to map and track traffic between an internal host and the Internet.
As you can see, the first letter in each acronym denotes the difference between NAT (Network Address Translation) and PAT (Port Address Translation), which should make it easier for you to remember which does what. Just remember that both NAT and PAT use at least one IP address and that PAT is also referred to as NAT overloading because it uses one IP address for all clients to multiple ports, whereas standard NAT uses a one-to-one IP address relationship per client.