Media Access Control (MAC) Addressing

The MAC address is the unique serial number burned into each network adapter that differentiates the network card from all others, just as your house number is unique on your street and identifies your home from all others. To be a part of any network, you must have an address so that others can reach you. Two types of addresses are found in a network: the logical (OSI model Layer 3, network) and the physical (OSI model Layer 2, data link). For this discussion of LAN environments, the physical address (also known as the Media Access Control [MAC] address) is relevant.

A MAC address is the physical address of the device. It is 48 bits (6 bytes) long and is made up of two parts: the organizational unique identifier (OUI) and the vendor-assigned address, as illustrated in Figure 5-1.

The MAC address on a computer might look like this: 00-08-a1-08-c8-13. This MAC address is used for the Fast Ethernet adapter on the computer in question. The OUI is 00-08-a1, and the vendor-assigned number is 08-c8-13.

The OUI is administered by the IEEE and identifies the vendor of the network adapter. The vendor-assigned portion of the MAC address is just that, the alphanumeric identifier assigned by the vendor. It is the combination of the OUI and the vendor-assigned number that ensures that no two network adapters have the same MAC address.

note

MAC addresses are represented as hexadecimal (hex) numbers.

With the hexadecimal numbering system, each half byte (4 bits) is assigned a hex digit, which is listed in Table 5-1, with its decimal and binary equivalents. Hex values are identified with an h or dollar sign, so $3E0, 3E0h, and 3E0H all stand for the hex number 3E0.