Network Address Translation (NAT)

Network Address Translation or NAT is a feature employed by routers that allows multiple internal private IP addresses to be mapped or translated to outside routable or public IP addresses. This way, multiple enterprises can use the same private IP address ranges internally, while remaining unique when accessing the public internet.

Port Address Translation or PAT is an extension of NAT that allows multiple private addresses to be mapped or translated to a single outside public address. The uniqueness of these translations is maintained by using a unique TCP or UDP port number for each internal communicating host. This allows a many-to-one or many-to-few relationship between internal non-routable private addresses and routable public addresses.

NAT and PAT have contributed substantially to the longevity of the IPv4 address space since they allow dozens or hundreds of hosts to share a single public IP address, mitigating against and further delaying IPv4 address exhaustion.