What Is an OUI? (And Its History)

What Is an OUI? (And Its History)

 By Charles Joseph | Cybersecurity Advocate
 Last update: November 25, 2023

An OUI, or Organizationally Unique Identifier, is a unique 24-bit number assigned to a specific organization by the Institute of Electrical and Electronics Engineers (IEEE).

OUIs are used to uniquely identify a hardware manufacturer or a specific organization within a network protocol, such as Ethernet or Wi-Fi.

History of OUIs

The history of OUIs dates back to the early days of Ethernet networking.

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In 1980, the original Ethernet specification, called the “Blue Book,” introduced the concept of a 48-bit MAC (Media Access Control) address, which is used to identify individual devices within a network.

The MAC address is divided into two parts: the first 24 bits, called the OUI, and the remaining 24 bits, which are used by the organization to create unique identifiers for their devices.

The IEEE Registration Authority, responsible for managing the assignment of OUIs, began operations in 1986.

Since then, OUIs have been used in various network protocols and standards, such as IEEE 802 (Ethernet, Wi-Fi, and Bluetooth), IEEE 1394 (FireWire), and others.

How Many OUIs Are Available?

The number of OUIs available is determined by the 24-bit length of the identifier.

With 24 bits, there can be a total of 16,777,216 (2^24) unique OUIs.

However, not all of these have been assigned, as the assignment is an ongoing process managed by the IEEE Registration Authority.

You can search for assigned OUIs and their corresponding organizations on the IEEE website.


OUIs play a crucial role in ensuring the uniqueness of MAC addresses, which is essential for the proper functioning of Ethernet and other network protocols.

By assigning OUIs to organizations, the IEEE ensures that each MAC address remains globally unique, preventing address conflicts within networks.

"Amateurs hack systems, professionals hack people."
-- Bruce Schneier, a renown computer security professional