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LAN Overview
139
Figure 4-4
MAC Address Format
The bit order in Ethernet is called little-endian; on FDDI and Token Ring, it is called big-endian.
Let's examine the meaning of these terms: On Ethernet, the most significant bit in a byte is
listed last in the byte. For example, assume that the binary string 01010101 is the value in a byte
of an Ethernet address. The right-most bit is considered to be the most significant bit in this
byte. However, if writing the same value in a byte of a Token Ring address, the value written
would be 10101010, so that the most significant bit is on the left. When bridging between
Ethernet and another type of LAN, the bit order in each byte of the MAC addresses must be
inverted. For example, the Token Ring address 4000.3745.0001 would be converted to
0200.ECA2.0080 before being sent onto an Ethernet.
The following list summarizes many of the key features of MAC addresses:
·
Unicast MAC addresses address an individual LAN interface card.
·
Broadcast MAC addresses address all devices on a LAN.
·
Multicast MAC addresses address a subset of the devices on an Ethernet or FDDI LAN.
·
Functional MAC addresses identify devices performing a specific IEEE-defined function,
on Token Ring only.
·
Ethernet orders the bits in each byte of the MAC address with the least significant bit first;
this convention is called little-endian.
·
Token Ring and FDDI order the bits in each byte of the MAC address with the most
significant bit first; this convention is called big-endian.
·
The most significant bit on the first byte of an address must have a value of binary 0 for
unicast addresses and 1 for broadcast, multicast, and functional addresses. This bit is
called the broadcast bit.
Most
Significant
Byte
Least
Significant
Byte
Ethernet - Most Significant Bit is last
Token Ring and FDDI - Most Significant
Bit is first
Vendor Code
(24 bits)
MAC
Address
ch04.fm Page 139 Monday, March 20, 2000 5:02 PM