Ethernet Address Formats
The IEEE intends for unicast addresses to be unique in the universe by administering the
assignment of MAC addresses. The IEEE assigns each vendor a code to use as the first 3 bytes of
its MAC addresses; that first half of the addresses is called the Organizationally Unique Identifier
(OUI). The IEEE expects each manufacturer to use its OUI for the first 3 bytes of the MAC
assigned to any Ethernet product created by that vendor. The vendor then assigns a unique value
in the low-order 3 bytes for each Ethernet card that it manufactures—thereby ensuring global
uniqueness of MAC addresses. Figure 1-5 shows the basic Ethernet address format, along with
some additional details.
Figure 1-5 Ethernet Address Format
Note that Figure 1-5 shows the location of the most significant byte and most significant bit in each
byte. IEEE documentation lists Ethernet addresses with the most significant byte on the left. However,
inside each byte, the leftmost bit is the least significant bit, and the rightmost bit is the most
significant bit. Many documents refer to the bit order as canonical; other documents refer to it as
little-endian. Regardless of the term, the bit order inside each byte is important for understanding the
meaning of the two most significant bits in an Ethernet address:
■ The Individual/Group (I/G) bit
■ The Universal/Local (U/L) bit
Table 1-5 summarizes the meaning of each bit.
1st Byte
1st Byte
2nd Byte 3rd Byte 4th Byte 5th Byte 6th Byte
I/G
Bit
OUI
Most
Significant
Bit
Most
Significant Byte
Least
Significant
Bit