MAC addresses on ethernets are 6 byte identifiers and are represented as a string of 12 characters. Here are some examples:
The MAC address can be used to uniquely identify a networked computer and it is used here at MIT to configure a computer dynamically. The dynamic host registration process records your computer's MAC address into a database so it is free to move about the MIT networks without having to reconfigure it.
If your computer's MAC address should change, you will need to re-register your computer. The MAC address will change if the computer itself is replaced or upgraded. It will also change if your computer uses a PCMCIA network interface and this PCMCIA card is replaced.