Editing MAC address (section)

===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.