Oh that's either if you're manually port forwarding your modem/router, else for your OSX firewall.
For OSX firewall settings, see OSX Firewall settings for Tiger/Leopard

As for your modem/router if it needs port forwarding, I might be able to help advise if it needs to be.
If you can supply these details: System details - help us to help you (click on blue link)

In the meantime, set the OSX firewall up or turn it off, also http://www.gnutellaforums.com/306179-post17.html