MAC address

In computer networking a Media Access Control address or MAC address is an identifier physically stored inside a network card or similar network interface and used to assign globally unique addresses in some OSI model Layer 2 networks, and in the physical layer of the Internet protocol suite. MAC addresses are assigned by the IEEE, and are used in many widely used network technologies, including (but not limiting to) the following:

• Ethernet
• Token ring
• 802.11 wireless networks
• ATM (switched virtual connections only, as part of an NSAP address)

Since the original designers of Ethernet had the foresight to use a 48 bit address space, there are potentially 2⁴⁸ or 281 million million possible MAC addresses. Ethernet MAC addresses are typically given as a string of 12 hexadecimal digits. The first six of these identify the manufacturer of the card, the last six are assigned by the manufacturer. The IEEE assigns the 24-bit OUI prefixes to organizations, effectively allocating blocks of 2²⁴ (around 16 million) MAC addresses at a time.

Changing MAC addresses

Although physical MAC addresses are permanent by design, several mechanisms allow modification of the MAC address that is reported by the operating system. This can be useful for privacy reasons, for instance when connecting to a Wi-Fi hotspot, or to ensure interoperability. Some ISPs bind their service to a specific MAC address; if the user then changes their network card or intends to install a router, the service won't work anymore. Changing the MAC address of the new interface will solve the problem. Similarly, some software licenses are bound to a specific MAC address. Changing the MAC address in this way is not permanent: after a reboot, it will revert to the MAC address physically stored in the card.

Under Linux, the MAC address of a Network Interface Card (NIC) can be changed using a command such as

ifconfig eth0 hw ether 00:01:02:03:04:05

(This needs to be done before network initialization.)

Under Windows XP, the MAC address can be changed in the Ethernet adapter's Properties menu, in the Advanced tab, as "MAC Address", "Locally Administered Address", or "Ethernet Address" (the exact name depends on the Ethernet driver used; not all drivers support changing the MAC address).

See also

• NSAP address, another endpoint addressing scheme.

External links

• IEEE OUI and Company_id Assignments: http://standards.ieee.org/regauth/oui/index.shtml