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Port address translation (PAT) is a function that allows multiple users within a private network to make use of a minimal number of IP addresses. Its basic function is to share a single IP public address between multiple clients who need to use the Internet publicly. It is an extension of network address translation (NAT).
Port address translation is also known as overload or port overload.
An example of PAT is a home network that is connected to the Internet. Within this setup, the system’s router is assigned a discrete IP address. Multiple users can access the Internet over the router, and are each assigned a port number as they do so.
PAT is used to give internal network hosts access to external network hosts. In a local area network (LAN) environment, many clients are accessing the Internet via the LAN’s router. Each client that demands Internet access is connected with public network hosts via a single public IP address. In this example, for outside or public networks, the LAN has a single identity, represented by a single IP address assigned to the whole LAN. However, the router assigns a specific port number for each client on the LAN/internal network in order to differentiate them. When clients on the LAN send data over a public network, it is transmitted from a single public IP address. After the completion of the requested task, the data/packet will return to the router and be distributed to the appropriate client on the basis of that client's port number.