Port Address Translation (PAT) is a technique for translating port numbers. To reduce the number of IP addresses in a corporate network, the IP addresses are mapped with the port numbers in the router. This allows a private local area network to get by with a single public IP address.
With the PAT technique, the computers in the corporate network cannot be dialed directly from the outside, since all LAN clients appear externally under one IP address and have only one common access to the Internet. Therefore, the port number is appended to the IP address for internal differentiation of the various servers. Thanks to this technique, incoming data packets can be forwarded by the router to the correct copmputer.