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
KEY POINT
Most Significant Byte
Least Significant Byte
Most Significant Byte
Least Significant Byte
|
1st Byte |
2nd Byte |
3rd Byte |
4th Byte |
5th Byte |
6th Byte |
Vendor-Assigned
Vendor-Assigned
Least Significant Bit
Most Significant Bit
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 noncanonical; 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:
Table 1-5 summarizes the meaning of each bit.
|
Field |
Meaning |
|
I/G |
Binary 0 means the address is a unicast; Binary 1 means the address is a multicast or broadcast. |
|
U/L |
Binary 0 means the address is vendor assigned; Binary 1 means the address has been administratively assigned, overriding the vendor-assigned address. |
The I/G bit signifies whether the address represents an individual device or a group of devices, and the U/L bit identifies locally configured addresses. For instance, the Ethernet multicast addresses used by IP multicast implementations always start with 0x01005E. Hex 01 (the first byte of the address) converts to binary 00000001, with the most significant bit being 1, confirming the use of the I/G bit.
NOTE Often, when overriding the MAC address to use a local address, the device or device driver does not enforce the setting of the U/L bit to a value of 1.
Continue reading here: VLAN Configuration
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