Suppose you have a local network, and you have two web servers on the network, Web Server 1 on 192.168.1.10 and Web Server 2 on 192.168.1.20. Suppose you also want an external client to be able to connect to either of these servers.

Only one IP is associated with the network, the external IP of the router: 1.2.3.4 in the schematic above. In order for a client to be able to communicate with both web servers, we can forward a port on the router to port 80 on Web Server 1 and forward a different port on the router to port 80 Web Server 2. So, we might forward port 8001 on the router to Port 80 on Web Server 1 and 8002 on the router to Port 80 on Web Server 2. This way, the external client could connect to Web Server 1 at http://1.2.3.4:8001/ and Web Server 2 at http://1.2.3.4:8002/