A MAC address is a 6-byte (48-bit) physical address that’s assigned to a device when it’s made. It is a globally unique identifier for the device. MAC addresses or also sometimes called burned-in addresses (BIA).
The first three bytes of the MAC address make up the organizationally unique identifier (OUI), which is a unique identifier assigned to the manufacturer. The last three bytes are unique to the device itself.
MAC addresses are written as 12 hexadecimal characters.