Editing MAC address (section)

==Address details==
[[File:MAC-48 Address.svg|right|330px|thumb|The structure of a 48-bit MAC address. The b0 bit distinguishes [[multicast]] and [[unicast]] addressing and the b1 bit distinguishes universal and locally administered addressing.]]

The [[IEEE 802]] MAC address originally comes from the [[Xerox Network Systems]] Ethernet addressing scheme.<ref name="Ieee802arch">
{{cite book
  | publisher = The Institute of Electrical and Electronics Engineers, Inc. (IEEE)
  | title = IEEE Std 802-2001
  | date = 2002-02-07
  | isbn = 978-0-7381-2941-9
  | url = http://standards.ieee.org/getieee802/download/802-2001.pdf
  | archive-url = https://web.archive.org/web/20030429223052/http://standards.ieee.org/getieee802/download/802-2001.pdf
  | url-status = dead
  | archive-date = April 29, 2003
  | access-date = 2011-09-08
  | quote = The universal administration of LAN MAC addresses began with the Xerox Corporation administering Block Identifiers (Block IDs) for Ethernet addresses.
  | page = 19
}}
</ref> This [[48-bit computing|48-bit]] address space contains potentially 2<sup>48</sup> (over 281 trillion) 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 (until 2080) for applications using EUI-48 space and restricts applications accordingly. The IEEE encourages adoption of the more plentiful EUI-64 for non-Ethernet applications.<ref>{{Cite web |title=Use of EUI-64 for New Designs |url=https://www.ieee802.org/secmail/msg00396.html |access-date=2024-07-07 |website=www.ieee802.org}}</ref>

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]].<ref name="eui tutorial" /><ref>{{Cite web|url=https://standards.ieee.org/faqs/regauth/|title=IEEE-SA - IEEE Registration Authority|website=[[IEEE]]|access-date=2018-09-20}}</ref> The IEEE now considers ''MAC-48'' to be an obsolete term.<ref name="mac-48">{{cite web |url=https://standards.ieee.org/products-services/regauth/oui36/index.html |title=MAC Address Block Small (MA-S) |access-date=2019-02-24 |archive-date=2021-04-18 |archive-url=https://web.archive.org/web/20210418090358/https://standards.ieee.org/products-services/regauth/oui36/index.html |url-status=dead }}</ref>  ''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.<ref name="eui tutorial" />{{efn|To convert a MAC-48 into an EUI-64, copy the OUI, append the two [[octet (computing)|octets]] {{MACaddr|FF-FF}} and then copy the organization-specified extension identifier. To convert an EUI-48 into an EUI-64, the same process is used, but the sequence inserted is {{MACaddr|FF-FE}}.<ref name="eui tutorial"/> In both cases, the process could be trivially reversed when necessary. Organizations issuing EUI-64s were cautioned against issuing identifiers that could be confused with these forms.}} These translations have since been deprecated.<ref name="eui tutorial">{{cite web|url=https://standards.ieee.org/wp-content/uploads/import/documents/tutorials/eui.pdf|title=Guidelines for Use of Extended Unique Identifier (EUI), Organizationally Unique Identifier (OUI), and Company ID (CID)|website=IEEE Standards Association|publisher=IEEE|access-date=5 August 2018}}</ref>

The [[Individual Address Block]] (IAB) is an inactive registry which has been replaced by the ''MA-S'' (''MAC address block, small''), previously named ''OUI-36'', and has no overlaps in addresses with the IAB<ref name=":0">{{Cite web|url=https://standards.ieee.org/faqs/regauth/|title=IEEE-SA - IEEE Registration Authority|website=[[IEEE]]|access-date=2018-11-27}}</ref> registry product as of January 1, 2014. The IAB uses an OUI from the ''MA-L'' (''MAC address block, large'') registry, previously called the ''OUI'' registry. The term ''OUI'' is still in use,<ref name=":0" /> but the IEEE Registration Authority does not administer them. An OUI is concatenated with 12 additional IEEE-provided bits (for a total of 36 bits), leaving only 12 bits for the organisation owning the IAB to assign to its (up to 4096) individual devices. An IAB is ideal for organizations requiring not more than 4096 unique 48-bit numbers (EUI-48). Unlike an OUI, which allows the assignee to assign values in various number spaces (for example, EUI-48, EUI-64, and the various context-dependent identifier number spaces, as in [[Subnetwork Access Protocol|SNAP]] or [[Extended Display Identification Data|EDID]]), 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.<ref>{{Cite web|url=https://standards.ieee.org/faqs/regauth/|title=IEEE-SA - IEEE Registration Authority|website=[[IEEE]]|access-date=2018-09-20}}</ref>

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 (while the registrant of an IAB cannot assign an EUI-64). MA-S does not include assignment of an OUI.

Additionally, the ''MA-M'' (''MAC address block, medium'') provides both 2<sup>20</sup> EUI-48 identifiers and 2<sup>36</sup> 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.

{{anchor|Universally administered address|Locally administered address}}

===Universal vs. local (U/L bit)===

Addresses can either be universally administered addresses (UAA) or locally administered addresses (LAA). A universally administered address is uniquely assigned to a device by its manufacturer. The first three octets (in transmission order) identify the organization that issued the identifier and are known as the [[organizationally unique identifier]] (OUI).<ref name="eui tutorial" /> The remainder of the address (three octets in EUI-48 or five in EUI-64) 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 (assigning the value of 1 to) the second-[[least significant bit|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.<ref name="mac-bit-field-names">
{{Cite web
  | url=https://networkengineering.stackexchange.com/a/69847
  | title=Ethernet frame IG/LG bit explanation - Wireshark
  | website=networkengineering.stackexchange.com
  | access-date=2021-01-05}}</ref>{{Self-published inline|date=November 2022}}{{Ref RFC|4291}}{{rp|p=20}} 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 {{MACaddr|06-00-00-00-00-00}}, the first octet is 06 (hexadecimal), the binary form of which is 000001'''1'''0, where the second-least-significant bit is 1. Therefore, it is a locally administered address.<ref name="IEEE Tutorial">{{cite web|url=https://standards.ieee.org/wp-content/uploads/import/documents/tutorials/macgrp.pdf|title=Standard Group MAC Addresses: A Tutorial Guide|publisher=IEEE-SA|access-date=2018-09-20}}</ref> Even though many  [[hypervisor]]s manage dynamic MAC addresses  [[MAC address#Universal addresses that are administered locally|within their own OUI]], often it is useful to create an entire unique MAC within the LAA range.<ref>{{cite web|url=https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/6/html/virtualization_administration_guide/sect-virtualization-tips_and_tricks-generating_a_new_unique_mac_address|title=Generating a New Unique MAC Address|publisher=RedHat|access-date=2020-06-15}}</ref>

==== 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 (with Digital Equipment Corporation's OUI AA-00-04) is administered locally. The DECnet software sets the last three bytes of the complete MAC address to {{MACaddr|00-xx-yy}} (so that the full MAC address is {{MACaddr|AA-00-04-00-xx-yy}}), where {{MACaddr|xx-yy}} reflects the host's DECnet network address ''xx.yy''. This eliminates the need for DECnet to have an  [[Address Resolution Protocol|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.<ref name="mac-bit-field-names" />{{Self-published inline|date=November 2022}}{{Ref RFC|4291}}{{rp|p=20}} When this bit is 0 (zero), the [[frame (networking)|frame]] is meant to reach only one receiving [[network interface controller|network interface]].<ref>{{cite web|url=https://standards.ieee.org/wp-content/uploads/import/documents/tutorials/fibre.pdf|title=Guidelines for Fibre Channel Use of the Organizationally Unique Identifier (OUI)|publisher=IEEE-SA|access-date=2018-10-11}}</ref> This type of transmission is called [[unicast]]. A unicast frame is transmitted to all nodes within the [[collision domain]]. In a modern wired setting (i.e. with ''[[Network switch|switches]]'', not simple ''[[Ethernet hub|hubs]]'') 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 [[network switch|switch]] does not know which port leads to a given MAC address, the switch will forward a unicast frame to all of its ports (except the originating port), an action known as [[unicast flood]].<ref>{{Cite web|url=https://supportforums.cisco.com/document/68421/overview-layer-2-switched-networks-and-communication|title=Overview of Layer 2 Switched Networks and Communication {{!}} Getting Started with LANs {{!}} Cisco Support Community {{!}} 5896 {{!}} 68421|website=supportforums.cisco.com|access-date=2016-05-17|date=2011-07-23}}</ref>{{Self-published inline|date=November 2022}} Only the node with the matching hardware MAC address will (normally) 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 (i.e. the second hexadecimal digit is odd) 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|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 {{MACaddr|FF:FF:FF:FF:FF:FF}}. A broadcast frame is [[flooding (computer networking)|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 [[Multicast address#Ethernet|instances]] of all four possibilities.<ref name="IEEE Tutorial" /> [[IPv6#Multicasting|IPv6 multicast]] uses locally administered, multicast MAC addresses in the range {{MACaddr|3'''<u>3</u>'''-33-xx-xx-xx-xx}} (with both bits set).{{Ref RFC|7042|rsection=2.3.1}}

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:

{| class="wikitable"
|+ Universal/local and individual/group bits in MAC addresses
! {{diagonal split header|I/G|U/L}}
! Universally administered
! Locally administered
|-
! Unicast (individual)
|{{MACaddr|x'''<u>0</u>'''-xx-xx-xx-xx-xx}}{{newline}}{{MACaddr|x'''<u>4</u>'''-xx-xx-xx-xx-xx}}{{newline}}{{MACaddr|x'''<u>8</u>'''-xx-xx-xx-xx-xx}}{{newline}}{{MACaddr|x'''<u>c</u>'''-xx-xx-xx-xx-xx}}
|{{MACaddr|x'''<u>2</u>'''-xx-xx-xx-xx-xx}}{{newline}}{{MACaddr|x'''<u>6</u>'''-xx-xx-xx-xx-xx}}{{newline}}{{MACaddr|x'''<u>a</u>'''-xx-xx-xx-xx-xx}}{{newline}}{{MACaddr|x'''<u>e</u>'''-xx-xx-xx-xx-xx}}
|-
! Multicast (group)
|{{MACaddr|x'''<u>1</u>'''-xx-xx-xx-xx-xx}}{{newline}}{{MACaddr|x'''<u>5</u>'''-xx-xx-xx-xx-xx}}{{newline}}{{MACaddr|x'''<u>9</u>'''-xx-xx-xx-xx-xx}}{{newline}}{{MACaddr|x'''<u>d</u>'''-xx-xx-xx-xx-xx}}
|{{MACaddr|x'''<u>3</u>'''-xx-xx-xx-xx-xx}}{{newline}}{{MACaddr|x'''<u>7</u>'''-xx-xx-xx-xx-xx}}{{newline}}{{MACaddr|x'''<u>b</u>'''-xx-xx-xx-xx-xx}}{{newline}}{{MACaddr|x'''<u>f</u>'''-xx-xx-xx-xx-xx}}
|}

===IEEE 802c local MAC address usage===

IEEE standard 802c<ref name="IEEE 802c-2017 tutorial">{{cite web|url=https://mentor.ieee.org/802-ec/dcn/17/ec-17-0174-00-00EC-ieee-802-tutorial-of-2017-11-06-local-mac-addresses-in-the-overview-and-architecture-based-on-ieee-std-802c.pdf|title=Local MAC Addresses in the Overview and Architecture based on IEEE Std 802c|publisher=IEEE-SA|access-date=2023-10-04}}</ref> further divides the locally administered MAC address block into four quadrants.  This additional partitioning is called Structured Local Address Plan (SLAP) and its usage is optional.

{| class="wikitable"
|+ SLAP quadrants for unicast local MAC addresses
! MAC address
! Quadrant name
! Identifier
! Usage
|-
! {{MACaddr|x'''<u>a</u>'''-xx-xx-xx-xx-xx}}
| Extended local
| ELI
| Assigned by IEEE, but uses a unique 3-octet company ID (CID) instead of an OUI.
|-
! {{MACaddr|x'''<u>e</u>'''-xx-xx-xx-xx-xx}}
| Standard assigned
| SAI
| For use in the forthcoming IEEE P802.1CQ specification, to be assigned dynamically by the Block Address Registration and Claiming (BARC) protocol.
|-
! {{MACaddr|x'''<u>2</u>'''-xx-xx-xx-xx-xx}}
| Administratively assigned
| AAI
| Can be randomly or arbitrarily assigned to devices.
|-
! {{MACaddr|x'''<u>6</u>'''-xx-xx-xx-xx-xx}}
| ''Reserved''
| ''Reserved''
| Reserved for future use, but may be used similarly to AAI until an IEEE specification utilizes this space.
|}