(Media Access Control)- A unique identifier assigned to Network interface cards, it also contains organizational (manufacturer) information and configuration parameters. Switches/routers(working at the 2nd-layer of the OSI model) use these unique address to deliver frames to the appropriate interface. Composed of six octets, FF:FF:FF:FF:FF:FF being the final address(reserved as the broadcast address), it has a 48bit address space capable of 281,474,976,710,656 unique permutations. See also: IEEE, 802, Ethernet, OSI