What is the Difference Between NAT and PAT?

By Stephen Bucaro

The main purpose for NAT (Network Address Translation) is to hide internal network IP addresses from the external Internet for security reasons. It works by mapping a pool of public IP addresses one-to-one to the private internal IP addresses, keeping the port number intact.

Internal Address	External Address
10.10.10.1:80	63.63.63.1:8000
10.10.10.2:80	63.63.63.1:8001
10.10.10.3:80	63.63.63.1:8002
10.10.10.4:80	63.63.63.1:8003

A port number identifies a specific protocol or process on a host to which a message or datagram is to be delivered. PAT (Port Address Translation) uses a single public IP address and maps multiple inside addresses to it by using different port numbers.

Most routers actually use NAPT (Network Address and Port Translation), which translates not only IP addresses, but also TCP and UDP port numbers.

An additional advantage of NAT is that by allowing multiple computers to connect to the Internet through a single external IP address it reduces the cost for connecting to the Internet. It also helped to extend the life of dwindling number of IPv4 addresses until IPv6 could be implemented.

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