Originally posted by bobw
From MacFixIt;
Troubleshooting iTunes 4: Playing songs over the Internet; Faster playing speed; International support; more
Playing songs over the Internet One of the most unheralded new features of iTunes 4 is the ability to share your playlists not just via Rendezvous, but also directly over the Internet. If you're behind a firewall, simply make sure the port #3689 is open on your router, pointing to your machine. Then go into your preferences and allow other users to connect to your machine. Then, your friends can pull down from "Advanced" to "Connect to Shared Music..." One caveat, however, is that you can't listen to other people's purchased music via Rendezvous (or the "connect to shared music"... feature).
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For Airport, open the Airport Admin Utility. Click Configure and click the Port Mapping Tab and add the port in there.