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ARP / RARP is commonly used to map the Layer 2 MAC address to an address in a Layer 3 protocol such as Internet Protocol (IP). On broadcast networks such as Ethernet the MAC address allows each host to be uniquely identified and allows frames to be marked for specific hosts. It thus forms the basis of most of the Layer 2 networking upon which higher OSI Layer protocols are built to produce complex, functioning networks.


ADDRESS DETAILS

The original IEEE 802 MAC address, now officially called "MAC-48", comes from the Ethernet specification. 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.

All three numbering systems use the same format, and differ only in the length of the identifier. The first three Octet s (in transmission order) identify the organization which issued the identifier, and are known as the Organizationally Unique Identifier (OUI). The following three (MAC-48 and EUI-48) or five (EUI-64) octets are assigned by that organization in nearly any manner they please, subject to the constraint of uniqueness. The IEEE expects the MAC-48 space to be exhausted no sooner than the year 2100 ; EUI-64s are not expected to run out in the foreseeable future.

MAC addresses that are permanently attached to a product by the manufacturer are known as " Corp. The last three octets represents the serial number assigned to the adapter by the manufacturer.

The following technologies use the MAC-48 identifier format:

The distinction between EUI-48 and MAC-48 identifiers is purely semantic: MAC-48 is used for network hardware; EUI-48 is used to identify other sorts of devices and software. (Thus, by definition, an EUI-48 is not in fact a "MAC address", although it is syntactically indistinguishable from one and assigned from the same numbering space.)

Note: The IEEE now considers the label MAC-48 to be an obsolete term which was previously used to refer to a specific type of EUI-48 identifier used to address hardware interfaces within existing 802 based networking applications and should not be used in the future. Instead, the term EUI-48 should be used by manufacturers and others in the field for this purpose.

EUI-64 identifiers are used in:
  • FireWire

  • IPv6 (as the low-order 64 bits of a unicast network address when temporary addresses are not being used)

  • 802.15.4 wireless personal-area networks


The IEEE has built in several special address types to allow more than one Network Interface Card to be addressed at one time:
  • The broadcast address, all one bits, is received by all stations on a Local Area Network . In Hexadecimal this would be "FF:FF:FF:FF:FF:FF".

  • Multicast Addresses , used with both Ethernet and FDDI, are received by stations on a LAN which have been configured to do so. Multicast addresses have the Least Significant Bit of their first octet set to one (value 01 in printed format.)

  • Locally Administered addresses are assigned by the network administrator instead of the hardware vendor. Locally Administered addresses have the second bit of their first octet set to one (value 02 in printed format.)

  • Functional Address es identify one of more Token Ring NICs that provide a particular service, defined in IEEE 802.5 .


In addition, the EUI-64 numbering system encompasses both MAC-48 and EUI-48 identifiers by a simple translation mechanism. To convert a MAC-48 into an EUI-64, copy the OUI, append the two octets 'FF-FF', and then copy the organization-specified part. To convert an EUI-48 into an EUI-64, the same process is used, but the sequence inserted is 'FF-FE'. In both cases, the process can be trivially reversed when necessary. Organizations issuing EUI-64s are cautioned against issuing identifiers which would be confused with these forms. The IEEE's policy is to discourage new uses of 48-bit identifiers in favor of the EUI-64 system.

Confusingly IPv6 -- one of the most prominent standards that uses EUI-64 -- applies these rules inconsistently. Due to an error in the appendix to the specification of IPv6 addressing, it is currently standard practice in IPv6 to extend MAC-48 addresses (such as IEEE 802 MAC address) to EUI-64 using 'FF-FE' rather than 'FF-FF'; it remains to be seen how this inconsistency will be resolved in the future.


PRINTED FORMAT

The standard (IEEE 802) format for printing MAC-48 addresses in human-readable media is six groups of two hexadecimal digits, separated by hyphens (-) in transmission order, e.g. 01-23-45-67-89-ab. This form is also commonly used for EUI-64. Other conventions include six groups of two separated by colons (:), e.g. 01:23:45:67:89:ab; or three groups of four Hexadecimal digits separated by dots (.), e.g. 0123.4567.89ab; again in transmission order.


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 Internet Service Provider s 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 by doing the following:
(You must be root in order for this to work)

:/etc/init.d/networking stop
: ifconfig eth0 hw ether 02:01:02:03:04:08
:/etc/init.d/networking start

NOTE: You cannot do this if using a DSL modem.

In Red Hat Linux and other similar systems ( Fedora Core , etc) an easy way to make it "permanent" across reboots is to just add a variable like this to your ifcfg-eth0 or similar file:

:MACADDR=12:34:56:78:90:ab

(upper or lower case on the MAC address are fine, because the network function does a "toupper" on it)

and service network restart for prompt results.

In Debian just put
: hwaddress ether 02:01:02:03:04:08
in the appropriate section of /etc/network/interfaces so that the MAC address is set when the network device is started.

You can also use the tool MACChanger to change the MAC address under Linux.

To change MAC address during boot time with MACChanger,add the following line to your /etc/network/interfaces

:pre-up macchanger -m 12:34:56:78:90:AB eth0


FreeBSD

Under FreeBSD , the MAC address can be changed in a similar way:

: ifconfig fxp0 ether 02:01:02:03:04:05

(This can be done without needing to take the interface down and back up)


OpenBSD


As of OpenBSD 3.8, the MAC address can be changed as follows:

: ifconfig bge3 lladdr 02:01:02:03:04:05


Mac OS X

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

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

: sudo ifconfig en0 ether 02:01:02:03:04:05

This must be done as the superuser and only works for the computer's Ethernet card. Instructions on spoofing AirPort Extreme (2.0) cards are available here . There are not, as of yet, any known ways to spoof original AirPort (1.0) cards.

The AirPort Extreme MAC address can also be changed easily with SpoofMac, available here: http://ungeord.net/smat/de/spoofmac/showData/


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", "Ethernet Address" or "Network 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 settings for each network interface can be found. The contents of the string value called 'NetworkAddress' will be used to set the MAC address of the adapter when next it is enabled. Resetting the adapter can be accomplished in script with the freely available command line utility devcon from Microsoft, or from the adapters context menu in the Network Connections control panel applet.

There is a nice tool to change the MAC address for all cards (even those that can't be changed through the adapter's Properties menu):
SMAC MAC Address Changer

Note: to check your MAC address easily on a Windows XP box, go to Run, type CMD, then type "ipconfig /all" without quotation in the command prompt. The number under physical address is the MAC address. If multiple IP are displayed, you should look under the label "Ethernet adapter x", where x is the name of your connection (which is Local Area Connection by default).

MAC address is 48 bits in length.


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.


Change The MAC Address Permanently

Download diagnostic programs for your card from this link .
Compile and run under Linux .
:cc -O -Wall -o rtl8139-diag rtl8139-diag.c
:./rtl8139-diag -w -H 12:34:56:78:9a:bc

Under Linux:
:ifconfig hw


Under FreeBSD:
:ifconfig link



SEE ALSO

  • NSAP Address , another endpoint addressing scheme.

  • , which allows multiple routers to share one IP Address and MAC address to provide router redundancy. The OpenBSD project has implemented an open source alternative to this, the Common Address Redundancy Protocol ( CARP ) protocol.



EXTERNAL LINKS