Question about TCP/IP in SE5

[Lohr]

Well, to make my question more clearly:
How do we get a game running with TCP/IP through the Internet?
We don't sitting both in the same town, and want to play SE5 from time to time for a couple of hours.

I know, there is my internal IP (192.168.0.xx) an the external IP, which the router uses to connect to the Internet.
Which Port(s) do we have to foreward in our routers? Are there multiple ones?
Do the Ports have to have different in my and my friends Computer/Router?
And i know, i have to use the external IP to get to connected to the host.
But what is with the IP's?