The 24 bits of the OUI code are the first three octets of the MAC address, which are reserved for manufacturer identification. Each manufacturer has its own OUI codes by which it can be identified. In this address range, each manufacturer can use the following 24 bits for individual identification of the network card.Protocol Identifier (PID).
In order to guarantee unique address assignment worldwide, IEEE has extended the address range for MAC addresses from 48 bits to 64 bits with the Extended Unique Identifier (EUI). The corresponding format is called EUI-64.