You may have more IP addresses assigned to the Internet interface, and run multiple services inside of your network that you want to make accessible from the Internet.
5xWWW servers scenario
As an example let's consider that you want to run 5 Web servers, where each of them has a separate domain associated with a different IP address.
Windows networking will allow you to bind additional IP addresses to a single interface. WinRoute requires that you do this before creating mappings for each IP address.
In such a scenario you will assign 5 IP addresses to your external Interface (linking to the Internet) and run web servers on other computers within your Internal network on private IP addresses such as 10.x.x.x or 192.x.x.x.
Each Web server may run on a separate computer or you may assign more IP addresses to one computer on your internal network and run all web servers on such a computer.
Then you will define 5 port mappings in the Port Mapping dialog. For each web server (domain) you will define: