IEEE MAC Address
LAN topologies following IEEE standards (IEEE LANs as well as ANSI FDDI) use 6-bytes MAC addresses (= station IDs, node IDs). The first three bytes contain a manufacturer code, the last three bytes contain a unique station ID. The IEEE standard allows 2-bytes MAC addresses as well; but nowadays these are unused.
MAC addresses have different binary format related to transmission direction: MSB vs. LSB, canonical vs. noncanonical.
MSB <> LSB / canonical <> noncanonical
Ethernet transmit octets with the least significant bit first (LSB mode, canonical mode)
Token-Ring and FDDI transmit octets with the most significant bit first (MSB mode, noncanonical mode).
Ethernet bridges that are connected to Token-Ring or FDDI networks have to face this fact.
MAC address parts
Manufacturer ID
The first three bytes contain a manufacturer code, the last three bytes contain a unique station ID.Manufacturer IDs are globally assigned (administered) by the IEEE. Because IEEE releases new manufacturer IDs if needed, protocol analyzers may not identify all manufacturer codes.
See:
These burned-in addresses can be replaced by driver or network management instruction. These so-called logical addresses are only valid for the current session and have to be configured again each time the adapter starts operation.
If logical addresses are set via the adapter's LAN driver, the second bit of the first byte of the source address is set to '1' (U/L bit).
Logical addresses are mostly used in Token-Ring networks. DECnet uses logical addresses in Ethernet networks, too.
Since logal addresses are assigned by the LAN administrator, duplicate MAC addresses may occur. Ethernet doesn't have any check for duplicate addresses as Token-Ring has (DAT = duplicate address test). Duplicate MAC addresses may cause severe errors.
The first bits of a MAC address are used as I/G bit and U/L bit.
I/G bit and U/L bit
The first two bits of a destination address convey certain information:
I/G = 0
Individual address: the destination is a singe node.
I/G = 1
Group address: the destination is a group of LAN nodes (multicast or broadcast address).
U/L = 0
Universally administered address: the adapter uses its burned-in MAC address.
U/L = 1
Locally administered address: the adapter uses a logical address (assigned by network administrator). U/L=1 may result in a hex code of 0x02 in the first byte. The U/L bit is always set when a logical address is assigned (even if the assigned address doesn't follow this convention). Therefore, it is impossible to imitate a burned-in address; but other logical address may be imitated at any time.
Source addresses don't use the I/G bit (because multiple stations cannot be the source of a single frame). The first bit of a source address doesn't have any special meaning in Ethernet LANs; in Token-Ring LANs, it is used as RII bit (RII = routing information indicator). The RII bit indicates that source routing information will follow the source address.
Token-Ring / logical adresses:
Destination Address (6 bytes) (Token-Ring)
Source Address (6 bytes) (Token-Ring)