OUI, MA-L, MA-M and MA-S Explained

The 48-bit MAC address

A conventional MAC address contains 48 bits, written as 12 hexadecimal digits. Each hexadecimal digit represents four bits. A registry assignment fixes the leading part of that address and leaves the remaining bits for the assignee to allocate to interfaces.

MA-L and the traditional OUI

An MA-L assignment fixes the first 24 bits. This is the familiar six-hexadecimal-digit prefix often called an OUI. A block beginning 001122, for example, covers addresses from 00:11:22:00:00:00 through 00:11:22:FF:FF:FF.

Historically, many lookup databases stored only this six-digit value. That remains useful for compatibility, but it cannot express every modern registration precisely.

MA-M: a 28-bit assignment

MA-M fixes seven hexadecimal digits and leaves 20 address bits for the assignee. A prefix such as 0011223 applies only to the portion of the broader 001122 range whose next nibble is 3. It therefore identifies a smaller allocation more precisely than the six-digit record.

MA-S: a 36-bit assignment

MA-S fixes nine hexadecimal digits and leaves 12 bits, or 4,096 addresses. It is suitable for organisations that need a smaller globally unique allocation. Because nine hexadecimal digits end halfway through the fifth displayed octet, MA-S prefixes are often easier to compare after removing separators.

CID records

A Company ID (CID) is also a unique 24-bit identifier, but IEEE states that it cannot be used to generate universally unique MAC addresses. It is intended for contexts that need a company identifier without a MAC address block. CID records are retained in this service’s source registry for reference and should not be interpreted as device-vendor address ranges.

Why longest-prefix matching matters

Suppose the database contains a 24-bit prefix 001122, a 28-bit prefix 0011223, and a 36-bit prefix 001122345. The address 00:11:22:34:5F:00 matches all three beginnings, but the 36-bit record is the most specific and should win.

macvendors.co.uk checks 36 bits first, then 28, then 24. The browser tool and API therefore use the narrowest available assignment. The API retains its established six-character mac_prefix response field for client compatibility even when a longer registry prefix supplied the vendor result.

Prefix length is not device identity

A more specific prefix improves the registration match; it does not reveal the exact product. One block can cover many device models, and one product line can use interfaces from several registered organisations. Use the result alongside DHCP, switch, Wi-Fi and inventory evidence.