2.1. Identifying a MAC Address and Its Parts

A unique Media Access Control (MAC) address is "burned into" each network interface card (NIC) (pronounced "nick") by the manufacturer of the NIC. The MAC address is a six-byte hexadecimal address that consists of two distinct parts. Each byte of the address consists of two hexadecimal digits. Bytes are separated by a hyphen (-). The first three bytes identify the manufacturer that created the NIC. The last three bytes should be unique for each NIC that the manufacturer produces. The purpose of this unique address is to identify the NIC, and there-fore the computer in which it is installed. Identifying each computer uniquely allows computers and software to address packets directly to that computer. Other NICs will then identify the packets as not being intended for the computer to which they are installed. In this section, we discuss how to identify a MAC address on a computer and the two components associated with the MAC address.

2.1.1. Critical Information

You should understand that the MAC address is also referred to as a physical address of a component. This is partially because it's usually burned into the device at the factory. Every NIC has its own unique MAC address that identifies it on the network. When it is installed in a computer, it becomes the unique identifier for that computer in the network. You should also understand that a MAC address has two distinct parts, and you should know what each part signifies.

2.1.1.1. Identifying a MAC Address

You can identify the MAC address of your local Windows 98, Windows NT, Windows 2000, Windows XP, or Windows Server 2003 computer by typing ipconfig /all at the command prompt. The physical address that is listed in the output is the MAC address. As you can see in Figure 2.1, the MAC address of the computer shown is 00-05-1B-00-4B-F6. On a Linux computer, you should use a similar command called ifconfig, which shows configuration information and allows you to change it as well.

Figure 2.1. An ipconfig /all output showing the physical (MAC) address of a computer

2.1.1.2. Components of a MAC address

A MAC address is a six-byte hexadecimal address. Hexadecimal numbers are used because of the tremendous number of combinations of addresses that are possible and yet the relative ease of which the number can be read. (A six-byte MAC address can yield 281,474,976,710,655 addressing possibilities.) The first three bytes of the MAC address indicate the manufacturer of the NIC (in Figure 2.1, 00-05-1B is an Organizational Identifier [OI] for Belkin). (Some manufacturers use more than one OI.) The last three bytes are a unique number from that manufacturer. This address should be unique in the world!

2.1.2. Exam Essentials

Know what a MAC address is and how to identify the MAC address on a computer. A MAC address is a hexadecimal address that is burned into every NIC by the manufacturer. The MAC address is also referred to as a physical address, which you can identify on a local computer by typing ipconfig /all on a command line.

Know the components of a MAC address. A MAC address is a six-byte hexadecimal address that uniquely identifies a NIC (pronounced "nick") and its manufacturer. The first three bytes represent the manufacturer of the NIC and the last three bytes should be unique to that manu-facturer's Organizational Identifier (OI).