overnet, isn't that from the makers of edonkey2000? i have no clue what stl is but opening a port is always the same procedure. if you have a router you must go to your configuration setup. your manual will tell you how to access it.
there would be an option to forward ports using a range of numbers. you must set the last part of the IP address to the machine you want to open it on, example 192.168.1.100.
your application will tell you which port range to open. there are 2 types, UDP and TCP. if you have any hardware or software firewalls you must open the same ports there also. this inlcudes the winXP firewall.
once you have done those you may need to specify the port number you have opened for the program.