The IEEE OUI Registry: MA-L, MA-M and MA-S Explained
Looking up a MAC address and getting a vendor name back depends on a public allocation registry maintained by the IEEE Registration Authority. Understanding how it is structured explains several things that puzzle people during lookups: why some prefixes are six hex digits while others need seven or nine, and why a single company can own more than a thousand prefixes.
How the 48 bits are divided
A standard MAC address is 48 bits. Traditionally the first 24 bits identify the manufacturer and the remaining 24 bits are assigned by that manufacturer to individual devices. Those first 24 bits are the OUI, or Organizationally Unique Identifier.
A 24-bit OUI comes with a 24-bit device space, which is roughly 16.7 million addresses. For most companies that is far more than they will ever need, so the IEEE later introduced finer-grained allocation tiers.
Three registration tiers
| Tier | Prefix assigned | Addresses available | Typical holder |
|---|---|---|---|
| MA-L (large) | 24 bits | ~16.78 million | Apple, Cisco, Huawei and other large vendors |
| MA-M (medium) | 28 bits | ~1.04 million | Mid-sized equipment makers |
| MA-S (small) | 36 bits | 4,096 | Low-volume products, module suppliers |
MA-L is the classic OUI. MA-M splits one 24-bit block into 16 parts, and MA-S splits it into 4,096. The important consequence is that a single 24-bit prefix may be shared by several different companies, so you have to look at more bits to tell them apart.
That is exactly why a lookup tool has to perform longest-prefix matching. This site tries to match at the 24-, 28- and 36-bit boundaries and returns the most specific hit, which is why entering a full address on the lookup page is usually more accurate than entering only the first three bytes.
Why large vendors hold thousands of prefixes
Apple has over 1,500 prefix records in the database and Cisco over 1,200. The reason is straightforward. Each MA-L supplies only 16.78 million addresses, and the cumulative shipment volumes of these companies exceed a single block many times over. Add product-line separation and blocks inherited through acquisitions, and the count grows steadily. You can see how many prefixes each company has accumulated in the vendor directory.
CIDs and locally administered addresses
Alongside those tiers the IEEE also issues Company IDs, or CIDs. A CID has the same shape as an OUI but cannot be used to build globally unique MAC addresses. It is intended for protocol identifiers and similar uses. A CID always has the locally administered bit set in its first byte, so it can never collide with a globally unique address.
For the full meaning of those two special bits in the first byte, see locally administered versus universal addresses.
Where the data comes from
The IEEE publishes its allocations as freely downloadable files. In practice the more commonly used source is the manuf file maintained by the Wireshark project, which normalises and supplements the raw IEEE data. That file is the source for this site and is synchronised automatically every 30 days.
Keep in mind that IEEE records identify the registrant, not necessarily the brand on the device in your hand. Prefixes registered by contract manufacturers and module suppliers end up in products badged by someone else. That is one of the common reasons a lookup returns nothing or an unexpected name.