Apple KB Article: Ports used by Apple Software

OS X security is a hot topic as of late, what with the virus contests, the security updates, and general rock solid reputation of the operating system. Truth be told the only totally safe computer, whether it be a Mac or a Windows machine, is one that is locked in a room and not connected to a network of any sort.

This completely safe computer is of very little use though, so most people keep their computers out where they can be used, and connect them to the internet. With this in mind I recommend that people use a firewall (the built in firewall in OS X is good, but a hardware firewall is even better).

The Apple Knowledge Base has an article entitled "Well Known" TCP and UDP Ports Used By Apple Software Products which has a great list of port numbers that you might want to make sure your firewall isn't blocking, or set up port forwarding for.

If you want to hunker down behind a firewall, but still want to share your iTunes music open up port 3689.