Any Minecraft experts out there, specifically on the Mac? I have a question about LAN games and ports.

I'm much more versed on the Mac than on Windows, so I want my boys to play Tekkit on the Mac side. I got the latest Technic launcher downloaded and running, and the game is actually (surprisingly) working pretty well. However, in order for them to play together on LAN, I have to disable the Mac's firewall completely. I'd rather not do that. I've noticed that regardless of the Mac (I've got three for them to play on), the LAN game is always opened on the same port. Does anybody know a way to open just that specific port for LAN games? Or do I need to do that on my router? I've searched some forums but have not found a way to do it. If it's a Command Line issue, I'm comfortable with that.