Information Technology Reference
In-Depth Information
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.
Ethernet
Today's Ethernet networks are based on the Ethernet development by Digital, Intel, and Xerox
(DIX). Version 1 of the standard was created in 1980. It used unbalanced signaling, meaning
that a 0 is represented by 0 voltage, and a 1 is represented by a positive voltage. In 1982, version
2 of Ethernet was introduced. It added the heartbeat signal to transceivers and moved to
balanced signaling. Balanced signaling uses positive and negative voltages, which allow more
speed. The heartbeat signal is used as a link test pulse. Ethernet version 2 is the basis of today's
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