A media access control address (MAC address) is a unique identifier assigned to a network interface controller (NIC) for use as a network address in communications within a network segment.
Universal and Local (U/L bit)
- Universally administered addresses (UAA) is uniquely assigned to a device by its manufacturer.
- Locally administered address (LAA) is assigned to a device by software or a network administrator, overriding the burned-in address for physical devices.
Locally administered addresses are distinguished from universally administered addresses by setting (assigning the value of 1 to) the second-least-significant bit of the first octet of the address.
- If the bit is 0, the address is universally administered.
- If it is 1, the address is locally administered.
Unicast and Multicast (I/G bit)
The least significant bit of an address’s first octet is referred to as the I/G, or Individual/Group, bit.
- When this bit is 0, the frame is meant to reach only one receiving NIC (Unicast).
- When this bit is 1, it’s a Multicast address.