[moron4hire]

IIRC, you could do this by IP, also. I remember my best friend in college and I mapping out the addresses of all the computers in our main lab and using it to send messages between each other. Kept the map in a notebook so we always knew where to send to regardless of where we sat down. We also used it to troll the student in front of us who was looking at porn and playing games in the middle of class. It didn't get shut off until after we came back between semesters, so I don't think we ever explicitly got caught, other than network techs seeing someone was doing something.