MAC address

A MAC address is a unique identifier assigned to a network interface controller for use as a network address in communications within a network segment. This use is common in most IEEE 802 networking technologies, including Ethernet, Wi-Fi, and Bluetooth. Within the Open Systems Interconnection network model, MAC addresses are used in the medium access control protocol sublayer of the data link layer. As typically represented, MAC addresses are recognizable as six groups of two hexadecimal digits, separated by hyphens, colons, or without a separator.
MAC addresses are primarily assigned by device manufacturers, and are therefore often referred to as the burned-in address, or as an Ethernet hardware address, hardware address, or physical address. Each address can be stored in the interface hardware, such as its read-only memory, or by a firmware mechanism. Many network interfaces, however, support changing their MAC addresses. The address typically includes a manufacturer's organizationally unique identifier. MAC addresses are formed according to the principles of two numbering spaces based on extended unique identifiers managed by the Institute of Electrical and Electronics Engineers : EUI-48—which replaces the obsolete term MAC-48—and EUI-64.
Network nodes with multiple network interfaces, such as routers and multilayer switches, must have a unique MAC address for each network interface in the same network. However, two network interfaces connected to two different networks can share the same MAC address.

Address details

The IEEE 802 MAC address originally comes from the Xerox Network Systems Ethernet addressing scheme. This 48-bit address space contains potentially 2⁴⁸ possible MAC addresses. The IEEE manages the allocation of MAC addresses, originally known as MAC-48 and now called EUI-48 identifiers. The IEEE has a target lifetime of 100 years for applications using EUI-48 space and restricts applications accordingly. The IEEE encourages adoption of the more plentiful EUI-64 for non-Ethernet applications.
The distinctions between EUI-48 and MAC-48 identifiers are in name and application only. MAC-48 was used to address hardware interfaces within existing 802-based networking applications; EUI-48 is now used for 802-based networking and is also used to identify other devices and software, for example Bluetooth. The IEEE now considers MAC-48 to be an obsolete term. EUI-48 is now used in all cases. In addition, the EUI-64 numbering system originally encompassed both MAC-48 and EUI-48 identifiers by a simple translation mechanism. These translations have since been deprecated.
The Individual Address Block is an inactive registry which has been replaced by the MA-S, previously named OUI-36, and has no overlaps in addresses with the IAB registry product as of January 1, 2014. The IAB uses an OUI from the MA-L registry, previously called the OUI registry. The term OUI is still in use, but the IEEE Registration Authority does not administer them. An OUI is concatenated with 12 additional IEEE-provided bits, leaving only 12 bits for the organisation owning the IAB to assign to its individual devices. An IAB is ideal for organizations requiring not more than 4096 unique 48-bit numbers. Unlike an OUI, which allows the assignee to assign values in various number spaces, the Individual Address Block could only be used to assign EUI-48 identifiers. All other potential uses based on the OUI from which the IABs are allocated are reserved and remain the property of the IEEE Registration Authority. Between 2007 and September 2012, the OUI value 00:50:C2 was used for IAB assignments. After September 2012, the value 40:D8:55 was used. Owners of an already assigned IAB may continue to use it.
The MA-S registry includes, for each registrant, both a 36-bit unique number used in some standards and a block of EUI-48 and EUI-64 identifiers. MA-S does not include assignment of an OUI.
Additionally, the MA-M provides both 2²⁰ EUI-48 identifiers and 2³⁶ EUI-64 identifiers, the first 28 bits being assigned by IEEE. The first 24 bits of the assigned MA-M block are an OUI assigned to IEEE that will not be reassigned, so the MA-M does not include assignment of an OUI.

Universal vs. local (U/L bit)

Addresses can either be universally administered addresses or locally administered addresses. A universally administered address is uniquely assigned to a device by its manufacturer. The first three octets identify the organization that issued the identifier and are known as the organizationally unique identifier. The remainder of the address are assigned by that organization in nearly any manner they please, subject to the constraint of uniqueness. A locally administered address is assigned to a device by software or a network administrator, overriding the burned-in address of a physical device.
Locally administered addresses are distinguished from universally administered addresses by setting the second-least-significant bit of the first octet of the address. This bit is also referred to as the U/L bit, short for Universal/Local, which identifies how the address is administered. If the bit is 0, the address is universally administered, which is why this bit is 0 in all UAAs. If it is 1, the address is locally administered. In the example address, the first octet is 06, the binary form of which is 00000110, where the second-least-significant bit is 1. Therefore, it is a locally administered address. Even though many hypervisors manage dynamic MAC addresses within their own OUI, often it is useful to create an entire unique MAC within the LAA range.

Universal addresses that are administered locally

In virtualisation, hypervisors such as QEMU and Xen have their own OUIs. Each new virtual machine is started with a MAC address set by assigning the last three bytes to be unique on the local network. While this is local administration of MAC addresses, it is not an LAA in the IEEE sense.
A historical example of this hybrid situation is the DECnet protocol, where the universal MAC address is administered locally. The DECnet software sets the last three bytes of the complete MAC address to , where reflects the host's DECnet network address xx.yy. This eliminates the need for DECnet to have an address resolution protocol since the MAC address of any DECnet host can be determined from its DECnet address.

Unicast vs. multicast (I/G bit)

The least significant bit of an address's first octet is referred to as the I/G, or Individual/Group, bit. When this bit is 0, the frame is meant to reach only one receiving network interface. This type of transmission is called unicast. A unicast frame is transmitted to all nodes within the collision domain. In a modern wired setting the collision domain usually is the length of the Ethernet cabling between two network interfaces. In a wireless setting, the collision domain is all receivers that can detect a given wireless signal. If a switch does not know which port leads to a given MAC address, the switch will forward a unicast frame to all of its ports, an action known as unicast flood. Only the node with the matching hardware MAC address will accept the frame; network interfaces with non-matching MAC-addresses ignore the frame unless they are in promiscuous mode.
If the least significant bit of the first octet is set to 1 the frame will still be sent only once; however, network interface controllers will choose to accept or ignore it based on criteria other than the matching of their individual MAC addresses: for example, based on a configurable list of accepted multicast MAC addresses. This is called multicast addressing.
The IEEE has built in several special address types to allow more than one network interface card to be addressed at one time:

Packets sent to the broadcast address, all one bits, are received by all stations on a local area network. In hexadecimal the broadcast address would be. A broadcast frame is flooded and is forwarded to and accepted by all other nodes.
Packets sent to a multicast address are received by all stations on a LAN that have been configured to receive packets sent to that address.
Functional addresses identify one or more Token Ring NICs that provide a particular service, defined in IEEE 802.5.

These are all examples of group addresses, as opposed to individual addresses; the least significant bit of the first octet of a MAC address distinguishes individual addresses from group addresses. That bit is set to 0 in individual addresses and set to 1 in group addresses. Group addresses, like individual addresses, can be universally administered or locally administered.

Ranges of group and locally administered addresses

The U/L and I/G bits are handled independently, and there are instances of all four possibilities. IPv6 multicast uses locally administered, multicast MAC addresses in the range .
Given the locations of the U/L and I/G bits, they can be discerned in a single digit in common MAC address notation as shown in the following table:

	Universally administered	Locally administered
Unicast
Multicast

IEEE 802c local MAC address usage

IEEE standard 802c further divides the locally administered MAC address block into four quadrants. This additional partitioning is called Structured Local Address Plan and its usage is optional.

MAC address	Quadrant name	Identifier	Usage
	Extended local	ELI	Assigned by IEEE, but uses a unique 3-octet company ID instead of an OUI.
	Standard assigned	SAI	For use in the forthcoming IEEE P802.1CQ specification, to be assigned dynamically by the Block Address Registration and Claiming protocol.
	Administratively assigned	AAI	Can be randomly or arbitrarily assigned to devices.
	Reserved	Reserved	Reserved for future use, but may be used similarly to AAI until an IEEE specification utilizes this space.