networking locally

Hello fellow javaheads,

I am wanting to write a program that networks with multiple computers on the same home network, but I am not sure how to go about it.

example:
lets say 5 people all are in 1 room together at home, they all download the program, then i want them to be able to login to the program and have it connect through the local network.

I know about networking when you put a server on a website and everyone connects through the server on the site.
or when you have it on a computer you use "localhost" and you can connect locally just within that computer.

but what would you use to connect within a local network?

I assume you would build a server in the program and then have each user connect to the 1 "main" user, but I am not sure on how this would be done?

Does anyone know of any tutorials or videos on how this would be done? I have looked but I don't really know what keywords to use to find it, all I am finding is tut's on how to network through a hosted site

Thank you for any help on this,
Bob