When I was in junior high, my friends and I discovered that a Command Prompt command called net send was enabled on our school network. AIM and similar services were blocked, but if we wanted to send messages to each other we could open up the Command Prompt and type something like: net send lab2126-24 test and get an alert that looked like: The computer names were predictable, something like lab + room number + computer number, and the computer numbers were physically written on the machines themselves.