1. Every piece of Ethernet hardware has a unique number assigned to it called it's MAC address. Remember that Ethernet is used locally to connect you to the Internet, and you share the local network with many other people. The MAC address is used by your local Internet router in order to direct your traffic to you rather than somebody else in your local area. Key point: The MAC address is 6-bytes long, and must be unique. In order to guarantee uniqueness, equipment vendors are assigned a unique 3-byte prefix, and they then assign their own 3-byte suffix. Thus, the first 3-bytes of a MAC address identifies what kind of hardware you have (3Com, Cisco, Intel, etc.). Key point: The uniqueness property of MAC addresses has interesting implications. It was an important clue in tracking down David Smith (the Melissa author). From Hacking-Lexicon