The MAC address, also called Ethernet address, is a unique, unmistakable address for all workstations, servers or computers connected to an Ethernet. It is a 48-bithardware address that is used to uniquely identify a node on the network.
The MAC address is a layer 2 address. It consists of a manufacturer identifier and an adapter identifier. The MAC address is part of the network card, it is stored in a PROM, cannot be changed and is usually printed on it. The 24-bit manufacturer identifier of the MAC address, which is the Organization Unique Identifier( OUI), is assigned and managed by the IEEE. Each manufacturer has its own OUI code. New manufacturers are registered by IEEE and can request their address mappings. Part of the following address range can be assigned for private persons or small companies. This 12-bit address range is the Individual Address Block( IAB).
The least significant bit( LSB) of the first byte of the manufacturer identifier is used to distinguish between an individual address and a group address, Individual/Group (I/G). If the I/G bit is set to 0, it is an individual address for unicast, if the I/G bit is set to 1, it is a multicast address.
The seventh bit is the U/L bit, which stands for Universal/Local. It is used to determine whether the MAC address is universal or locally managed, and locally assigned addresses must not be involved in cross-network communication. If the U/L bit is set to 0 the address has a unique company identifier. It is then a Universally Administrated Addresses( UAA), which is administered by the IEEE. With a 0 the address is locally administered. It is then a Locally Administered Address( LAA). The other bits of the first byte are used as well as the remaining 3 bytes for the adapter identification. The second 24 bits are assigned and managed by the respective manufacturer.
In order to guarantee an unambiguous address assignment worldwide, IEEE has extended the address range for the MAC addresses from 48 bits to 64 bits with the Extended Unique Identifier( EUI). The corresponding format is called EUI-64.