It would be quite logical to assume that if you are going to act as a server, your computer is the server and others connect to it. Kaillera's site only registers new servers so that they appear on the serverlist. If you just want to play with your friend (as in, the server you made is not public) you need to give him/her your IP address so he'll know where to connect. Of course, if you decide to make it public, kaillera stores the server IP (your IP) and others connect to it. Naturally, if you don't keep your server active, others can't connect to it.