Why invert the 7th bit?

MAC addresses can be divided into two types:

• UAA (Universally Administered Address)

  • Uniquely assigned to the device by the manufacturer

• LAA (Locally Administered Address)

  • Manually assigned by an admin (with the mac-address command on the interface) or protocol. Doesn't have to be globally unique.

You can identify a UAA or LAA by the 7th bit of the MAC address, called the U/L bit (Universal/Local bit):

• U/L bit set to 0 = UAA

• U/L bit set to 1 = LAA

In the context of IPv6 addresses/EUI-64, the meaning of the U/L bit is reversed:

• U/L bit set of 0 = The MAC address the EUI-64 interface ID was made from was an LAA.

• U/L bit set to 1 = The MAC address the EUI-64 interface ID was made from was a UAA.