Organizationally Unique Identifier (OUI)

An Organizationally Unique Identifier (OUI) is a 24-bit identifier that the IEEE Registration Authority assigns to manufacturers and organisations on a globally unique basis. In an EUI-48 MAC address, the first three octets form the OUI, while the manufacturer defines the remaining three. This gives each network interface a unique hardware address.

OUIs are also used in EUI-64 identifiers and various network protocols. In network administration, an OUI lookup helps assign devices in ARP tables, DHCP logs or packet captures to a manufacturer. However, this is not possible with randomised or locally administered MAC addresses, which smartphones frequently use. In addition to OUIs, the IEEE assigns smaller address blocks such as MA-M and MA-S.