I do have a doubt about the joining without logging in, will there be a timer? Most likely I hope.
Because even if it isn't some higher level hacking by IP tracking and whatnot, there are those who have siblings at home, and as you know siblings from the same household often share the same IP, and as you also know siblings can be jerks and can cause problems if they find out how it works.
I'll touch on this pretty briefly, as Oli already had.
IP spoofing wouldnt allow a two way communication. For example, if I spoofed your address, all the data I send, would be from you, but all data received, would also go to you, so I would never see the server connect, because it would never report back to me, as shown in this majestic text.
[Crym] > [Tommy] >< [Server]