You can set a static UDP listening port and TCP handshake port by configuring it in System > Preferences > Settings > Advanced Settings under Peer Connections. The larger box can be left empty as it is only needed if you have multiple internet connections.
Note:
This will require you to forward that port's UDP/TCP traffic from your router to the machine, if you are behind a router. Follow the instructions for your router from PortForward.com. If you have multiple machines behind the same router to do this on, you will need to choose different ports for each to avoid conflicts.