My old modem

First you have to "create" an application, which in reality is nothing more than giving a particular port a name. I call this application port8795 and use port 8795.

After this you have to assign this application to a local device. We use the device I have called M5. Somewhere else in the modem interface, you can find that this device has IP and that is the home network IP of my tablet.

Go on your modem to where you can forward ports
Application Sharing
Press on New Application
New Application
Give a name
Specify port
Specify the port (8795)
Application Sharing
Press "Assign a game or application to a local network device"
Assign to M5
Select port8795 from Game or Application and the Device (here M5)
port8795 assigned to M5
port8795 assigned to M5
M5 information
Under the home network devices we see that M5 has the IP of my tablet,

External netwerk connections to port 8795 of the modem are now forwarded to port 8795 on