NAPT Static Address Translation

Basic NAPT Static Address Translation shows an example of a basic static NAPT translation. Client1 is a device on an internal private network that wants to connect to the web service at the Server1 IP address TCP port 80. The web service is in fact hosted by a Server1 on the public network. A static NAT translation is configured that maps the Client1 private network address to a static public network IP address and TCP port 80 to the Server1 public network IP address and TCP port 80.

A packet arrives at the NAT router from Client1 with the Client1 private IP address: port 35000, but leaves the NAT router with the public static source address: port 80. In both cases the destination is for Server1‘s public network IP address: port 80. From Server1‘s point of view, Client1‘s IP address is the public static IP address: port 80. Server1 doesn‘t know anything about its actual private Client1 IP address: port 35000.

When Server1 responds to Client1, its packet arrives at the NAT router with Client1‘s translated public static address: port 80 as the destination address, but leaves the NAT router with Client1‘s actual address: port 35000 as the destination address. The NAT router delivers Server1‘s response the Client1 IP address, port 35000.