I'm building a music library program, and I want to have the ability to share the library in the LAN. How can I discover others who share their library? I'd like to find others' libraries without typing in IPs and stuff.
Probably the easiest way is to use UDP to periodically send a message (containing some info advertising your library's presence) to the broadcast address. This will be received by all the hosts on your subnet (and perhaps further, depending on your router configuration). If your app listens for these messages from other hosts, it will over time be able to find all the other instances of your app on the subnet.
Edit: found this question which has answers that go into more specifics of what I'm talking about.