Hi.
Internal port should be the port number of the internal device which you are going to do port forwarding for. Internal Port accepts only 1 port. If you need to open a few ports for the internal device, you can add some more rules to open ports for your devices.
Service port is the external port for you to remotely access the internal device from outside. Service port can be the same with internal port. And they also can be different.
You can also check this link : http://forum.tp-link.com/showthread.php?59275-8970-port-forwarding&region=china

Thank you, but if I want to forward about 100 ports. I need to add 100 rules?

Hi shining01,

In general: yes, you need to set each port forwarding.
Alternatively you could use DMZ to forward all traffic on all external ports to one specific internal host (unsecure if you don't use a firewall).
Or you could use a UPnP-Software to set the portforwardings with less work than doing it manually in the GUI.

Regards,
Thomas Windscheif