MAC address

In computer networking a Media Access Control address or MAC address is a 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. 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. The IEEE hands out 24-bit OUI prefixes to organizations, effectively allocating blocks of 2²⁴ (around 16 million) MAC addresses at a time.

Although MAC addresses are permanent by design, several mechanisms allow their modification. As an example, Internet gateway routers allow the network administrator to set the WAN interface MAC address, to fool ISPs that bind their service to a specific NIC. Also, running Linux, one can arbitrarily set the MAC address to fool software license keys tied to specific NICs using the command

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

(This needs to be done before network initialization and is not permanent.)

See also: NSAP address for another endpoint addressing scheme.

External links

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