A universally administered address
is uniquely assigned to a device by its manufacturer; these are sometimes called "burned-in addresses" (BIA). The first three octets
(in transmission order) identify the organization that issued the identifier and are known as the Organizationally Unique Identifier
(OUI). The following three (MAC-48 and EUI-4
or five (EUI-64) octets are assigned by that organization in nearly any manner they please, subject to the constraint of uniqueness. The IEEE expects the MAC-48 space to be exhausted no sooner than the year 2100
; EUI-64s are not expected to run out in the foreseeable future.
A locally administered address is assigned to a device by a network administrator, overriding the burned-in address.
Locally administered addresses do not contain OUIs.
Universally administered and locally administered addresses are distinguished by setting the second least significant bit of the most significant byte of the address. If the bit is 0, the address is universally administered. If it is 1, the address is locally administered. The bit is 0 in all OUIs. For example, 02-00-00-00-00-01. The most significant byte is 02h. The binary is 00000010 and the second least significant bit is 1. Therefore, it is a locally administered address