What is Port Address Translation ? Explained with Examples

By | 9th November 2015

What is NAT Overload or Port Address Translation PAT.

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.

NAT overoad or PAT

NAT overoad or PAT2

Looking at the figure above, NAT overload or PAT used unique source port numbers on the inside global IP address to distinguish between translations. As NAT processes each packet, it uses a port number to identify the packet source – 2333 and 1555 in the above figure -.

* The source address (SA) is the inside local IP address with the assigned port number attached.

* The destination address (DA) is the outside local IP address with the service port number attached, in this case port 80: HTTP (Internet).

At the border gateway router (R1), NAT overload changes the SA to the inside global IP address of the client, again with the port number attached. The DA is the same address, but is now referred to as the outside global IP address. When the web server replies, the same path is followed but in reverse.

IP Addressing

IP Addressing Subnetting


Subnetting Class B Address

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