Simple, use the guide made by MS.

http://www.microsoft.com/athome/secu...ect/ports.mspx

(towards the buttom)

Do it on your server. on point 8 which says: In Name or IP address of the computer hosting this service on your network, type 127.0.0.1.

Put your INTERNAL IP (of the PC you want to play JJ2 on in there ) in instead. (to find it out, run this on your PC: Start > Run > ipconfig /all)

you need to open port :10052 once, for TCP.
and once again, for UDP.


then you should be done