MAC address |
In s on the names EUI-48 and EUI-64 . (The EUI stands for Extended Unique Identifier.)
Address Resolution Protocol/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 the layer 2 networking upon which higher OSI_model protocols 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 octets (in transmission order) identify the organization which issued the identifier, and are known as the Organisational 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.
MAC addresses permanently attached to a product by the manufacturer are known as burned-in addresses (BIA) or sometimes as Universally Administered Addresses (UAA). The BIA can be overridden with a Locally Administered Address (LAA). The following technologies use the MAC-48 identifier format: *Ethernet *Token ring *IEEE 802.11 wireless networks *Bluetooth *FDDI *Asynchronous Transfer Mode (switched virtual connections only, as part of an NSAP address) *SCSI and Fibre Channel (as part of a World Wide Name)
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.)
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)
The IEEE has built in several special address types to allow more than one Network Interface Card to be addressed at one time:
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 inconsistenly. 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 format for printing MAC-48 addresses in human-readable media is three groups of four hexadecimal digits, separated by dots (.), in transmission order; e.g., 0123.4567.89ab. However, very few products do this. The most common format is six groups of two hexadecimal digits, separated by colons (:) or hyphens (-), still in transmission order, as in 01:23:45:67:89:ab or 01-23-45-67-89-ab; this form is also commonly used for EUI-64. Also, sometimes used format as in 012345-6789ab.
= 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, or to ensure interoperability. Some internet service providers 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 00:01:02:03:04:08 :/etc/init.d/networking start
In Redhat and other similar systems (Fedora, 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.
==FreeBSD==
Under FreeBSD, the MAC address can be changed in a similar way:
: ifconfig fxp0 ether 00:01:02:03:04:05
(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 01:02:03:04:05:06
==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 00:01:02:03:04:05 or
: sudo ifconfig en0 ether 00:01:02:03:04:05
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 [http://www.suspekt.org here]. There are not, as of yet, any known ways to spoof original AirPort (1.0) cards.
==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_MACHINESYSTEMCurrentControlSetControlClass{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.
==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 =
= External links =
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