What is an OUI (Organizationally Unique Identifier)?
An OUI (Organizationally Unique Identifier) is the first 24 bits — the first 3 bytes, written as 3 hex pairs — of a MAC address, assigned by the IEEE Registration Authority to a specific manufacturer.
Every network interface (WiFi card, Ethernet port, Bluetooth radio) ships with a MAC address burned in at the factory. The IEEE splits that 48-bit address into two halves: the first 24 bits identify the manufacturer (the OUI), and the second 24 bits are a serial number the manufacturer assigns internally. Apple's OUIs are different from Samsung's, which are different from Netgear's — so the first half of a MAC address is effectively a manufacturer fingerprint.
This is what a MAC/OUI lookup actually does: it takes the first 3 bytes of an address and checks them against the IEEE's public assignment registry. "F4:F5:E8" maps to Google, Inc. because Google registered that block. The IEEE publishes the full assignment list, and it updates as manufacturers register new blocks or (rarely) return old ones.
One important exception: a randomized (private) MAC address, which modern phones use by default, doesn't have a real OUI at all — it's generated locally by the device, not assigned by the IEEE. A lookup on a randomized address correctly returns "no vendor," not a wrong answer.
FAQ
- Is an OUI the same as a MAC address?
- No. The OUI is the first 3 bytes (first half) of a MAC address. The full MAC address is 6 bytes — the OUI (manufacturer) plus a 3-byte device-specific serial assigned by that manufacturer.
- Who assigns OUIs?
- The IEEE Registration Authority. Manufacturers apply for and pay for an OUI block, then assign addresses from it to their own hardware.
- Can two different manufacturers share an OUI?
- No — each OUI block is assigned to exactly one organization. But one manufacturer can hold many OUI blocks (large manufacturers like Apple and Samsung have dozens).