NAT overload sometimes called PAT (Port Address Translation) maps multiple unregistered or private IP addresses to a single registered or public IP address by using different ports. This is what most home broadband routers do. Your ISP assigns an IP address to your router, but you find out that all the computers in the house could connect to the Internet at the same time.
PAT uses unique source port numbers on the inside global IP address to distinguish between translations
When a client logs on the Internet, the NAT router assigns a port number to its source address. NAT overload or PAT ensures that clients use a different TCP port number for each client session with a server on the Internet. When the server response, the client router routes the packet based on the source port number, which had become the destination port number. This process also validates that the incoming packets were requested, thus adding a degree of security to the session.
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