Understanding OUI & IEEE Registration

Updated February 17, 2026 · 5 min read

When you use our MAC Address Lookup tool, you're querying the IEEE OUI database — a public registry that maps MAC address prefixes to their manufacturers. But how does this system actually work, and how do companies get their addresses?

What is an OUI?

An OUI (Organizationally Unique Identifier) is a 24-bit (3-byte) prefix assigned to a company or organization by the IEEE. It forms the first half of a MAC address and uniquely identifies the manufacturer of a network device.

  3C:22:FB:xx:xx:xx
  ────────
  OUI = 3C:22:FB → Apple, Inc.

With an OUI, a manufacturer gets a block of 16,777,216 unique addresses (2²⁴) — the remaining 3 bytes they can assign however they like to individual devices.

IEEE Registration Tiers

Not every company needs 16 million addresses. The IEEE offers three tiers of MAC address blocks:

Registry	Prefix Size	Addresses	Cost (2026)
MA-L (Large)	24-bit (3 bytes)	16,777,216	~$3,885
MA-M (Medium)	28-bit (3.5 bytes)	1,048,576	~$1,940
MA-S (Small)	36-bit (4.5 bytes)	4,096	~$810

MA-L — The Classic OUI

This is what most people mean by "OUI." Large manufacturers like Apple, Samsung, and Intel hold dozens of MA-L blocks because they ship millions of devices per year. Apple alone has over 900 registered OUIs.

MA-M — Medium Block

For companies that need more than 4,096 but fewer than 16 million addresses. The prefix is 28 bits — the first 3 bytes plus the upper nibble of the 4th byte. This means our standard 3-byte OUI lookup will show the parent block owner, but the specific sub-assignment isn't publicly visible.

MA-S (formerly OUI-36) — Small Block

For small manufacturers, IoT device makers, or companies with limited product lines. Only 4,096 addresses per block, but much cheaper.

The CID — Company ID

There's also a CID (Company ID) — similar in format to an OUI but explicitly not for use as MAC addresses on public networks. CIDs are used for internal identification purposes within protocols and applications. They use the locally administered bit to avoid conflicts with real MAC addresses.

How Registration Works

1. Application: A company applies to IEEE through their Registration Authority
2. Payment: The company pays the one-time fee for their chosen tier
3. Assignment: IEEE assigns the next available prefix from the pool
4. Publication: The assignment is published in the public OUI database (unless the company requests "Private" listing)
5. Manufacturing: The company programs their assigned prefix into every device they manufacture

The OUI Database

The IEEE publishes the full OUI database as a free download at standards-oui.ieee.org. This is the same dataset that powers our lookup tool. As of early 2026, the database contains over 38,000 registered entries.

The database is updated regularly as new registrations come in. We refresh our local copy monthly to keep results current.

Interesting OUI Facts

• Xerox was assigned the very first OUI block (00:00:00) — they co-invented Ethernet
• Apple has more OUI registrations than almost any other company — over 900 blocks covering billions of possible addresses
• VMware has its own OUI (00:50:56) — that's how you can tell if a network interface belongs to a virtual machine
• 00:00:5E is reserved for IANA — used for VRRP virtual router MAC addresses and other protocol-specific purposes
• The 02:xx:xx range (locally administered bit set) is reserved for randomized/spoofed addresses and will never have a manufacturer lookup

Limitations of OUI Lookup

While OUI lookup is useful, it has some limitations:

• Private registrations won't show the company name
• MA-M and MA-S blocks may show the parent block rather than the specific sub-assignee
• ODM/OEM devices — many products are manufactured by third parties. Your "Brand X" router might show up as a Foxconn or Quanta device
• Randomized MACs won't match any OUI because they use the locally administered bit
• Very old or counterfeit devices may have unregistered or reused OUIs