Ethernet Addressing
At the Data Link Layer, Ethernet, Token Ring and FDDI all share the same addressing format of 48 bits. This, as many other things in networking, came from Xerox PARC, but is now administered by the IEEE. Think of this like a social security number and a name. I am certain there is another Donna Harrington in the world, but we do not share the same social security number. I don't think I'll capture my social security number for you in Sniffer just in case.
Figure 5-16 illustrates some examples of vendor codes. It is not my intent to make any snide vendor remarks in the case of duplicate addresses because even the well-known manufacturers have made mistakes. Take a look at Example 5-21 or one of your previous show interfaces command outputs to see that there is room for duplication on your part, too.
Example 5-21. Burned in Address (BIA)
r1#show interfaces ethernet 0 Ethernet0 is up, line protocol is up Hardware is Lance, address is 0000.0c8d.6705 (bia 0000.0c8d.6705) Description: r1e0 to hosta and hostb Internet address is 192.168.5.17/28 |
Example 5-21 shows the address and burned-in-address (BIA) to be one and the same. However, someone can use the mac-address [new mac address] command to change this on the interface. Obviously, this should be done with care because interfaces on the same LAN sharing the same MAC will not function properly. Perhaps you will get an opportunity to experience that sooner than you think. The format for the new MAC address is xxxx.xxxx.xxxx. However, MAC addresses are commonly displayed with dashes, spaces, or dots for readability, as follows:
xx-xx-xx-xx-xx-xx
xx xx xx xx xx xx
xxxx.xxxx.xxxx
Certain addresses are reserved for multicast purposes. Notice that the last bit of the first byte is set to 1 for the examples presented in Figure 5-17. This is actually the first bit of the 48-bit address as it is serially transmitted onto the medium. The bytes are sent left-to-right, but within each byte the bits are sent right-to-left. This is referred to as canonical order, as illustrated back in Chapter 1, “Shooting Trouble.”
