MAC Addresses

A MAC address is a unique 6-byte address that is burned into each network interface or more specifically, directly into the PROM chip on the NIC. The number must be unique, as the MAC address is the basis by which almost all network communication takes place. No matter which networking protocol is being used, the MAC address is still the means by which the network interface is identified on the network. Notice that I say network interface. That's very important, as a system that has more than one network card in it will have more than one MAC address.

MAC addresses are expressed in six hexadecimal values. In some instances, the six values are separated by colons (:); in others, hyphens (-) are used; and in still others, a space is simply inserted between the values. In any case, because the six values are hexadecimal, they can only be numbers 09 and the letters AF. So, a valid MAC address might be 00-D0-56-F2-B5-12 or 00-26-DD-14-C4-EE. There is a way of finding out whether a MAC address exists through the IEEE, which is responsible for managing MAC address assignment. The IEEE has a system in place that lets you identify the manufacturer of the network interface by looking at the MAC address.

For example, in the MAC address 00-80-C8-E3-4C-BD, the 00-80-C8 portion identifies the manufacturer and the E3-4C-BD portion is assigned by the manufacturer to make the address unique. The IEEE is the body that assigns manufacturers their IDs, called Organizationally Unique Identifiers, and the manufacturer then assigns the second half, called the Universal LAN MAC address. From the IEEE's perspective, leaving the actual assignment of addresses to the manufacturers significantly reduces the administrative overhead for the IEEE.

As discussed, MAC addresses are expressed in hexadecimal format. For that reason, they can only use the numbers 09 and the letters AF. There are only six bytes, so a MAC address should be six groups of two characters. Any other number of characters or any answer that contains a letter other than those described can be immediately discounted as an answer.

The method by which you can discover the MAC address of the network interfaces in your equipment depends on which operating system is being used. Table 3.5 shows you how to obtain the MAC address on some of the more common platforms.

Table 3.5. Commands to Obtain MAC Addresses

Platform	Method
Windows 95/98/Me	Run the winipcfg utility.
Windows NT/2000	Run ipconfig /all from a command prompt.
Linux/Some UNIX	Run the ifconfig -a command.
Novell NetWare	Run the config command.
Cisco Router	Run the sh int <interface name> command.

Be prepared to identify the commands used to view a MAC address as shown in Table 3.5. You might be asked to identify these commands on the Network+ exam.

As you work with network interfaces more, you might start to become familiar with which ID is associated with which manufacturer. Although this is a skill that might astound your friends and impress your colleagues, it won't help you with the Network+ exam. Just knowing what does, and doesn't, represent a valid MAC address will be sufficient on the exam.