Organization Unique Identifier (OUI) "Organization Unique Identifier" is a unique identifier issued to various organizations.
Among the 6-byte MAC addresses burned in any network card (NIC), the first 3 bytes represent OUI, which represents the manufacturing organization of NIC. Usually, this identifier is unique.
Ethernet is addressed by media access control (MAC) addresses, which are burned into each Ethernet interface card (NIC). The MAC address, also known as the hardware address, is a 48-bit (6-byte) hexadecimal format.
A 48-bit MAC address consists of two parts: a 24-bit Organization Unique Identifier (OUI) and the remaining 24-bit code assigned by the manufacturer.
Organization Unique Identifier (OUI) is assigned to an organization by Institute of Electrical and Electronics Engineers (IEEE) and contains 24 bits (3 bytes). Each organization assigns a global management address (24 bits or 3 bytes) in turn, which is unique for each network card produced by the manufacturer.
Among the 24-bit OUI, the highest bit is the individual/group (I/G) bit. When its value is 0, it can be considered that the address is actually the MAC address of the device, which may appear in the source address part of the MAC header. When its value is 1, it can be considered that the address represents the broadcast address or multicast address in Ethernet, or the broadcast address or function address in TR and FDDI. The next bit is the G/L bit (also called U/L, where U stands for global). When its value is 0, it represents a global management address (assigned by IEEE). When its value is 1, it means that the local address is governed in management (just like in DECnet).
The last 24-bit address of Ethernet indicates the code assigned by local management or manufacturer. This part of the address of the first network card manufactured by the manufacturer usually starts with 24 zeros, and the last network card ends with 24 1 (* * * has 16,777,216 network cards). In practice, it is found that many manufacturers use the same six-digit hexadecimal number as the last six digits of the same network card.