MAC Address Format

Ethernet or Token Ring router interfaces and all device network interface cards (NICs) are identified with a unique burned-in address (BIA). This is the MAC address, which is also called the physical address. It is an implementation of Layer 2 of the OSI reference model—or more specifically, the MAC layer of the IEEE model to identify the station. The MAC address is 48 bits in length (6 octets) and is represented in hexadecimal.

The router output in Example 4-1 shows the MAC address (00-10-7b-3a-92-3c) of an Ethernet interface.

Example 4-1 Router Interface MAC Address router> show interface

Ethernet0 is up, line protocol is up

Hardware is Lance, address is 0010.7b3a.923c (bia 0010.7b3a.923c)

Internet address is 135.119.110.30/24

MTU 1500 bytes, BW 10000 Kbit, DLY 1000 usec, reliability 255/255, txload 1/255, rxload 1/255 Encapsulation ARPA, loopback not set Keepalive set (10 sec) ARP type: ARPA, ARP Timeout 04:00:00

Last input 00:00:00, output 00:00:00, output hang never Last clearing of "show interface" counters never

Input queue: 1/75/0/0 (size/max/drops/flushes); Total output drops: 0 Queueing strategy: fifo Output queue :0/40 (size/max)

5 minute input rate 1000 bits/sec, 2 packets/sec 5 minute output rate 1000 bits/sec, 2 packets/sec

1999164 packets input, 379657585 bytes, 0 no buffer Received 1785091 broadcasts, 0 runts, 0 giants, 0 throttles 1 input errors, 0 CRC, 0 frame, 0 overrun, 1 ignored 0 input packets with dribble condition detected 745208 packets output, 82211652 bytes, 0 underruns 0 output errors, 63 collisions, 16 interface resets 0 babbles, 0 late collision, 345 deferred 0 lost carrier, 0 no carrier

0 output buffer failures, 0 output buffers swapped out

The first three bytes of a MAC address form the Organizational Unique Identifier (OUI), which identifies the manufacturer/vendor. The last three octets are administered by the manufacturer and assigned in sequence.

Canonical Transmission

When converting hexadecimal MAC addresses to binary, each hexadecimal number is represented in its 4-bit binary equivalent. For example, ac-10-7b-3a-92-3c is converted normally to binary as the following:

10101100 00010000 01101011 00111010 01010010 00111100

For Token Ring networks, each octet of this MAC address is transmitted from left to right, from the most significant bit (MSB) to the least significant bit (LSB). This noncanonical transmission is also known as MSB first.

For Ethernet networks, each octet of the previous MAC address is transmitted from left to right, but LSB to MSB. The difference is that for each octet, the LSB is transmitted first and the MSB is transmitted last. This canonical transmission is also known as LSB first. The MAC address AC-10-7b-3a-92-3c is transmitted as the following:

00110101 00001000 11010110 01011100 01001010 00111100

The octet AC is transmitted from left to right as 00110101, the second octet (10) is transmitted from left to right as 000010000, and so on.

For both Ethernet and Token Ring networks, the order of each octet is transmitted the same: from Most Significant Octet to Least Significant Octet. The difference is in the transmission order of the bits of each octet. For Ethernet networks, the LSB of the first octet is transmitted first. This bit is the Individual/Group (I/G) Address Bit. If the I/G is set to 0, it indicates an individual MAC address. If the I/G bit is set to 1, it indicates that the address is a group address. The I/G is set to 1 for broadcast and multicast MAC addresses.

The first (leftmost) bit in the binary representation is the I/G Address Bit. If set to 0, it indicates an individual address. It can be set to 1 in an address allocated by the vendor to indicate that the address is a group address. The second leftmost bit is the U/L bit. If set to 0, it indicates a universally administered address. If set to 1, it indicates that the address is locally administered.

Continue reading here: Ethernet

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