IEEE 802 address
In its basic form, an IEEE 802 address is a 48-bit or 64-bit address assigned to unique local area network interface cards, in which it is a "burnt-in address" (BIA). 48-bit is the current default length. A BIA is created during manufacture of the network interface card (NIC) or other hardware, and stored in read-only memory in the hardware. When a computer containing that NIC initializes, the default behavior is for the BIA address to become the active medium access control (MAC; i.e., "Layer 2 address") for the interface. Image: Basic IEEE 802 address format|thumb|left|Basic IEEE 802 address format; the OUI can be 24 or 40 bits long Layer 2 networks based on bridge (computer network)|bridging directly track active MAC addresses and make forwarding decisions based on them, but bridged networks have less flexibility and inferior resource utilization in comparison to router|routed networks. Bridged networks still can be viable in small networks, especially embedded ones without professional network administrators.
Basic structure
In the default form, the BIA is intended to be globally unique; it is also called a universally administered address. The high-order bits of such an address are assigned as a Organizationally Unique Identifier (OUI) to a hardware manufacturer; the manufacturer is responsible for maintaining uniqueness in the low-order bits below that OUI.
The OUIs in common use are 24 bits (i.e., 3 octets or bytes) in length, but IEEE Project 802 has defined a 40 bit OUI, making a 64 bit address. Extension to 64 bits was initially intended to ensure that the address space could not be exhausted, although the current 48 bit space should be adequate for the 21st century's needs. Internet Protocol version 6 Stateless Address Autoconfiguration (SLAAC), however, needs a 64 bit unique computer (or interface) identifier. SLAAC has a method for deriving 64-bit identifiers from conventional 48-bit identifiers, but it is not, as yet, clear if IPv6 may start driving the BIA standard to 48 bits. Much consideration would have to be given to backward compatibility.
Locally administered address
In some applications, more in the past than in current use, certain networking architectures required the MAC address to be set by some architecture-specific mechanism; the BIA would not be used when the interface initialized. Such architectures, mostly obsolete, include Banyan VINES, Digital Equipment Corporation's DECNET, IBM System Network Architecture (SNA), and Novell's IPX. The most common reason for algorithmic assignment is to do away with the need for the Address Resolution Protocol; the overhead of ARP has not been a practical problem. Image: Locally administered address|thumb|Location of G/L bit Locally-administered addresses (LAA) are distinguished from globally unique addresses by the global/local (G/L) bit, which is the second least significant bit of the most significant byte of the entire address. A 1 value of this bit means the address is locally administered.
Multicast addressing
Another bit in the most significant byte indicates if the address is intended to go to one specific destination (i.e., unicasting), or to multiple destinations (i.e., multicasting). If the low-order bit of that byte is set to 1, the address is of a multicast group; if the bit is set to 0, it is a unicast address.