OUI (MAC address prefix)
The OUI is the first three bytes of a MAC address, assigned by the IEEE to a manufacturer, so it reveals who made a network interface.
What an OUI is
Every network interface, wired or wireless, has a hardware address called a MAC address: 48 bits, written as six pairs of hexadecimal digits such as 3C:22:FB:1A:2B:3C. The IEEE assigns the first three bytes, 3C:22:FB in that example, to a manufacturer, and the manufacturer assigns the remaining three bytes to individual devices. That first half is the Organizationally Unique Identifier. Because the registry is public, looking up the prefix tells you which company made the interface, or at least which company bought the block.
Smaller blocks exist for companies that need fewer addresses: MA-M assignments cover 28 bits and MA-S assignments 36 bits. A lookup tool checks all three registries.
Why it matters
MAC addresses stay on the local network. They are not sent across the internet, so a website cannot see yours; what it sees is your IP address, assigned by the router that performs NAT for your devices. Within the local network, though, the OUI is useful: identifying an unknown device on your Wi-Fi, confirming that a "phone" on the guest network is actually a laptop, auditing an inventory, or checking whether a cheap adapter carries the chipmaker's prefix or a counterfeit one.
For privacy, modern phones and laptops randomize the MAC address they use on each Wi-Fi network. A randomized address has the locally administered bit set (the second hex digit is 2, 6, A, or E) and does not match any manufacturer, which is why a lookup for it returns nothing.
How to check
The MAC address lookup tool accepts an address in any common format, with colons, hyphens, dots, or no separators, and returns the registered organization, the block type, and whether the address is unicast or multicast and globally or locally administered. The data comes from the IEEE registry, which we refresh monthly.
Common misconceptions
The OUI identifies the manufacturer of the network chip or the device vendor who bought the block, not necessarily the brand on the case. Many devices from different brands share an OUI because they use the same wireless module. And a MAC address cannot be geolocated or traced to an owner from the address alone; it is not registered to end users anywhere.