I have some noob problems

. I am new to network programming and still struggling with basic conceptions and terms. When i try to use my own pieces or the shipped examples with autojoin/sync i am able to connect several instances to a server all at the same machine. Over Lan i get some error message(in my respective language) that something is already in use and it can not connect.
The other examples work fine over lan, but i did`nt fully understand the lobby server information exchange procedure and what steps are necessary.
I hope someone can give me a hint.
(I'am sure it`s pretty basic stuff)