This chapter is from the book
This chapter is from the book
This chapter is from the book
MAC addresses
A MAC address is a unique 6-byte address that is burned into each network interface. 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 (0-9) and the letters A through F. 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.
CAUTION
As discussed, MAC addresses are expressed in hexadecimal format. For that reason they can only use the numbers 0-9 and the letters A-F. 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 3.4 shows you how to obtain the MAC address on some of the more common platforms.
CAUTION
Be prepared to identify the commands used to view a MAC address as shown in Table 3.4. You may be asked to identify these commands on the Network+ exam.
Table 3.4 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. |
As you work with network interfaces more and more, you may start to become familiar with which ID is associated with which manufacturer. Although this is a skill that may 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.