OUI
acronymStands for: Organizationally Unique Identifier
The 24-bit prefix the IEEE Registration Authority assigns to an organisation, forming the first half of the MAC addresses it issues.
An MA-L block brings an OUI and 2^24 addresses; the smaller MA-M and MA-S blocks share their first 24 bits with other buyers, so a 24-bit lookup cannot name their owner.
The IEEE's guidelines define the OUI as 'a 24-bit (three octet) sequence' and sell address space in three sizes: an MA-L gives 2^24 EUI-48 identifiers from an assigned OUI; an MA-M gives 2^20 and 'does not include an OUI assignment'; an MA-S gives 2^12 from a 36-bit prefix. The assignments are public and searchable, which is how a lookup turns a prefix into a company.
Before reading an OUI as a vendor, read the two lowest bits of the first octet. If the group bit is set, the address is multicast or broadcast; if the local bit is set, the address was set by software, and the bits where an OUI would be mean nothing. Randomised Wi-Fi addresses, the default on current phones, are local by design, so the honest result of looking one up is no vendor at all.
Also known as: organizationally unique identifier, ma-l, ma-m, ma-s, vendor prefix, manufacturer prefix