What are a universal address and a local administered address?
A MAC address is called a universally administered address (UAA) when it's set by the manufacturer or a locally
administered addresses (LAA) when it's assigned by the system administrator.
For a universally administered address, 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-48) or five (EUI-64) octets are assigned by that organization in nearly any manner possible,
subject to the constraint of uniqueness.
The IEEE has a target lifetime of 100 years for applications using MAC-48 space, but it encourages the adoption
of EUI-64s instead.
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. 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 administered universally. If it is 1, the address is administered locally. In the
example address 06-00-00-00-00-00 the most significant byte is 06 (hex), the binary form of which is 00000110,
where the second least significant bit is 1. Therefore, it is a locally administered address. Consequently, this
bit is 0 in all OUIs.