A unique number assigned to all network interface devices. It is used by many data link layer protocols (see seven-layer reference model) to distinguish and target the devices attached to a network. MAC addresses are 48- or 64-bit numbers that are divided into two parts. A unique three-byte Organizationally Unique Identifier (OUI) identifies the device’s manufacturer and must be purchased from the IEEE. The remaining three or five bytes are assigned by the manufacturer in any way it chooses, provided all instances are unique. 48-bit MAC addresses (called EUI-48 or MAC-48) are used by most network technologies, including Ethernet, token ring, and Bluetooth; 64-bit MAC addresses (called EUI-64) are used by IPv6 (see TCP/IP) and FireWire. MAC numbers may be overridden on their own responsibility by network administrators.