A MAC address, or Media Access Control address, is a 48- or 64-bit address associated with a network adapter. While IP addresses are associated with software, MAC addresses are linked to the hardware of network adapters. For this reason, the MAC address is sometimes called the hardware address, the burned-in address (BIA), or the physical address. MAC addresses are expressed in hexadecimal notation in the following format: 01-23-45-67-89-AB, in the case of a 48-bit address, or 01-23-45-67-89-AB-CD-EF, in the case of a 64-bit address. Colons (:) are sometimes used instead of dashes (-).
MAC addresses are often considered permanent, but in some circumstances, they can be changed. There are two types of MAC addresses:
Universally Administered Address
The UAA, or Universally Administered Address, is the most commonly used type of MAC address. This address is assigned to the network adapter when it is manufactured. The first three octets define the manufacturer, while the second three octets vary and identify the individual adapter. All network adapter manufacturers have their own code, called the Organizationally Unique Identifier (OUI). For example, in the MAC address 00-14-22-01-23-45, the first three octets are 00-14-22. This is the OUI for Dell. Other common OUIs include 00-04-DC for Nortel, 00-40-96 for Cisco, and 00-30-BD for Belkin. Most large manufacturers of networking equipment have multiple OUIs.
Locally Administered Address
The LAA, or Locally Administered Address, is an address that changes an adapter's MAC address. The LAA is a type of administered MAC address, and it is possible to change the LAA of a network adapter to any address of allowed length. When the LAA is set, the network adapter uses the LAA as its MAC address. Otherwise, the network adapter uses the UAA as its MAC address. All devices on the same subnet must have different MAC addresses, however. MAC addresses are very useful in diagnosing network issues, such as duplicate IP addresses, so it is a good practice to allow devices to use their UAAs instead of assigning LAAs, unless there is a compelling reason to do so.
MAC addresses are useful for security purposes, as they are only rarely changed from the default. IP addresses can change dynamically, especially on networks using DHCP to assign IP addresses, so MAC addresses are often a more reliable way to identify senders and receivers of network traffic. On wireless networks, MAC address filtering is a common security measure to prevent unwanted network access. In MAC address filtering, a wireless router is configured to accept traffic from certain MAC addresses. In this way, as whitelisted devices are assigned new IP addresses through DHCP, they retain their ability to communicate on the network. Any intruder attempting to impersonate a valid user on the network by masquerading with a faked IP address will not be able to do so because the computer's MAC address will not match any of those in the whitelist. This security method is only minimally successful, however, as a determined intruder can fake a MAC address almost as easily as an IP address.