Network Address Translation - Implementation - Translation of The Endpoint

Translation of The Endpoint

With NAT, all communication sent to external hosts actually contain the external IP address and port information of the NAT device instead of internal host IPs or port numbers.

• When a computer on the private (internal) network sends a packet to the external network, the NAT device replaces the internal IP address in the source field of the packet header (sender's address) with the external IP address of the NAT device. PAT may then assign the connection a port number from a pool of available ports, inserting this port number in the source port field (much like the post office box number), and forwards the packet to the external network. The NAT device then makes an entry in a translation table containing the internal IP address, original source port, and the translated source port. Subsequent packets from the same connection are translated to the same port number.

• The computer receiving a packet that has undergone NAT establishes a connection to the port and IP address specified in the altered packet, oblivious to the fact that the supplied address is being translated (analogous to using a post office box number).

• A packet coming from the external network is mapped to a corresponding internal IP address and port number from the translation table, replacing the external IP address and port number in the incoming packet header (similar to the translation from post office box number to street address). The packet is then forwarded over the inside network. Otherwise, if the destination port number of the incoming packet is not found in the translation table, the packet is dropped or rejected because the PAT device doesn't know where to send it.

NAT will only translate IP addresses and ports of its internal hosts, hiding the true endpoint of an internal host on a private network.