What is an OUI?

An OUI (Organizationally Unique Identifier) is a 24-bit number that the IEEE Registration Authority assigns to an organization, and it is the first three octets of a MAC address. An OUI is assigned as part of an MA-L block, which extends it with a further 24 bits, so an OUI identifies the registrant of an address block rather than the device that carries an address from it.

Where does an OUI appear in an address?

In an EUI-48 built from an OUI, the OUI is the initial and most significant three octets, so EC:0D:51:1A:2B:3C starts with the OUI EC:0D:51. That OUI is registered to Apple, Inc., and the whole block EC:0D:51:00:00:00–EC:0D:51:FF:FF:FF resolves to the same registrant. Look it up on the EC:0D:51 page, or check any address in the MAC address lookup.

What comes with an OUI?

An OUI is not sold on its own. Since 1 January 2014 it is assigned with an MA-L block, which gives the holder 2^24 EUI-48 identifiers and 2^40 EUI-64 identifiers built from that OUI. MA-L is also the current name for what was simply an OUI assignment before that date.

The smaller MA-S assignment works differently: it assigns an OUI-36, a 36-bit (nine hexadecimal digit) identifier, and does not include a 24-bit OUI of its own — the first 24 bits of an MA-S block are an OUI held by the IEEE Registration Authority itself. That is visible in a lookup: 70:B3:D5 resolves to the IEEE Registration Authority as an MA-L block, while 70:B3:D5:00:1 resolves to SOREDI touch systems GmbH as an MA-S block. The block sizes page covers the three assignment types.

Is an OUI the same as a MAC address?

No. An OUI is 24 bits and identifies an organization; a MAC address is 48 bits and identifies an interface, using the OUI as its first half. The two are related but not interchangeable: an OUI names a registrant, while a MAC address is one value out of the block that registrant holds.

What an OUI does not tell you

It does not identify a product. The registration record names the organization that registered the block, and a lookup returns that record — no model, no owner and no location. An address from an IEEE-held block such as 70:B3:D5 is the clearest example: the registrant in the record is the IEEE Registration Authority, not the maker of the device. Can a MAC address identify a device? explains the limits.

Look up an OUI

Enter the first six hexadecimal digits in the MAC address lookup, or query it programmatically with the MACLookup API.