MAC address

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In computer networking a media access control address (MAC address) is a code on most forms of networking equipment that allows for that device to be uniquely identified.

Address details

A 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 limited to) the following:

• Ethernet
• Token ring
• 802.11 wireless networks
• Bluetooth
• FDDI
• 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 281,474,976,710,656 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 and are known as the Organisational Unique Identifier (OUI), the last six are assigned by the manufacturer and are known as the burned-in addresses (BIA) or sometimes as the Universally Administered Addresses (UAA). The IEEE assigns the 24-bit OUI prefixes to organizations, effectively allocating blocks of 2²⁴ (16,777,216) MAC addresses at a time. The BIA can be overridden with a Locally Administered Address (LAA).

The IEEE has built in several special address types to allow more than one NIC to be addressed at one time:

• Broadcast address or FFFF.FFFF.FFFF (hexadecimal). This tells all NICs within the confines of a local area network (LAN) to process a frame with this MAC address.
• Multicast address, used with both Ethernet and FDDI. An address which has the lowest bit of the first byte set to '1'. A NIC will only accept a frame with a multicast MAC address if it is configured to do so.
• Functional address identify one of more Token Ring NICs that provide a particular service, defined by the IEEE in their standard ISO/IEC 8820-5.

Changing MAC addresses

Although physical MAC addresses are permanent by design, several mechanisms allow modification, or "spoofing", 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.

As a MAC address can be changed, it can be unwise to rely on this as a single method of authentication. IEEE 802.1x is an emerging standard better suited to authenticating devices at a low level.

Linux

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

or

ip link set eth0 address 00:01:02:03:04:05

(This needs to be done before network initialization.)

Mac OS X

Under Mac OS X, the MAC address can be altered in a fashion similar to the Linux method:

sudo ifconfig en0 lladdr 00:01:02:03:04:05

This must be done as the superuser, and requires that the interface be taken down and brought back up again to function as expected with the new MAC address. It remains to be determined whether this functionality is actually extant in Darwin at the present time.

Windows

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 in this way.

However, a better solution - requiring Administrative User Rights - is to pass over the System Registry Keys under HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Class\{4D36E972-E325-11CE-BFC1-08002BE10318}. Here each network Interface can be found back - where only a supplemental String Value has to be added in order to prepare the change of the MAC Address ...

Other systems

You can use a third-party utility to change the MAC of almost any Ethernet adapter - two of them are listed below in External Links.

Most consumer-grade routers allow for a user-specified MAC address to be given.

See also

• NSAP address, another endpoint addressing scheme.
• Cisco Hot Swappable Router Protocol (HSRP), which allows multiple routers to share one IP address and MAC address to provide network redundancy. The OpenBSD project has implemented an open source alternative to this, the Common Address Redundancy Protocol (CARP) protocol.

External links

• IEEE OUI and Company_id Assignments: http://standards.ieee.org/regauth/oui/index.shtml
• Ethereal's Ethernet vendor codes and well-known MAC address list: http://www.ethereal.com/distribution/manuf.txt
• Michael Patton's "Ethernet Codes Master Page": http://www.cavebear.com/CaveBear/Ethernet/
• SMAC (by KLC Consulting) is a Windows MAC Address Modifying utility for Windows 2000 & XP that has the ability to change any network adapter's address. It is available at: http://www.klcconsulting.net/smac
• Macshift (by Nathan True) is a Free Software Windows Scriptable MAC Address changer. It's available (with source code) at http://macshift.natetrue.com
• Very Useful Set of Instructions for changing the MAC Address under Windows 2000/XP/2003 - http://www.nthelp.com/NT6/change_mac_w2k.htm

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