MAC addresses are most often assigned by the manufacturer of a network interface card (NIC) and are stored in its hardware, the card's read-only memory, or some other firmware mechanism. If assigned by the manufacturer, a MAC address usually encodes the manufacturer's registered identification number. It may also be known as an Ethernet hardware address (EHA), hardware address, adapter address, or physical address.
MAC addresses are formed according to the rules of one of three numbering name spaces managed by the Institute of Electrical and Electronics Engineers (IEEE): MAC-48, EUI-48, and EUI-64. The IEEE claims trademarks on the names EUI-48 and EUI-64, in which EUI is an acronym for Extended Unique Identifier.
Although intended to be a permanent and globally unique identification, it is possible to change the MAC address on most of today's hardware, an action often referred to as MAC spoofing. Unlike IP address spoofing, where senders spoofing their address in a request direct the receiver into sending the response elsewhere, in MAC address spoofing the response is received by the spoofing party. However, MAC address spoofing is limited to the local broadcast domain.
A host cannot determine from the MAC address of another host whether that host is on the same link (network segment) as the sending host, or on a network segment bridged to that network segment.
In TCP/IP networks, the MAC address of an interface can be queried knowing the IP address using the Address Resolution Protocol (ARP) for Internet Protocol Version 4 (IPv4) or the Neighbor Discovery Protocol (NDP) for IPv6. On broadcast networks, such as Ethernet, the MAC address uniquely identifies each node on that segment and allows frames to be marked for specific hosts. It thus forms the basis of most of the Link layer (OSI Layer 2) networking upon which upper layer protocols rely to produce complex, functioning networks.
Notational conventions
The standard (IEEE 802) format for printing MAC-48 addresses in human-friendly form is six groups of two hexadecimal digits, separated by hyphens (-) or colons (:), in transmission order, e.g. 01-23-45-67-89-ab, 01:23:45:67:89:ab. This form is also commonly used for EUI-64. Another convention commonly used by networking equipment uses three groups of four hexadecimal digits separated by dots (.), e.g. 0123.4567.89ab; again in transmission order.Address details
All three numbering systems use the same format and differ only in the length of the identifier. Addresses can either be "universally administered addresses" or "locally administered addresses".
A universally administered address is uniquely assigned to a device by its manufacturer; these are sometimes called "burned-in addresses" (BIA). The first three octets (in transmission order) identify the organization that issued the identifier and are known as the Organizationally Unique Identifier (OUI).[2] 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;[3] EUI-64s are not expected to run out in the foreseeable future.
A locally administered address is assigned to a device by a network administrator, overriding the burned-in address. Locally administered addresses do not contain OUIs.
Universally administered and locally administered addresses are distinguished by setting the second least significant bit of the most significant byte of the address. If the bit is 0, the address is universally administered. If it is 1, the address is locally administered. In the example address 06-00-00-00-00-01 the most significant byte is 06 (hex), the binary form of which is 00000110, where the second least significant bit is 1. Therefore, it is a locally administered address.[4] Consequently, this bit is 0 in all OUIs.
If the least significant bit of the most significant octet of an address is set to 0 (zero), the frame is meant to reach only one receiving NIC.[citation needed] This type of transmission is called unicast. A unicast frame is transmitted to all nodes within the collision domain, which typically ends at the nearest network switch or router. Only the node with the matching hardware MAC address will accept the frame; network frames with non-matching MAC-addresses are ignored, unless the device is in promiscuous mode.[citation needed]
If the least significant bit of the most significant address octet is set to 1, the packet will still be sent only once; however, NICs will choose to accept it based on different criteria than a matching MAC address: for example, based on a configurable list of accepted multicast MAC addresses. This is called multicast addressing.
The following technologies use the MAC-48 identifier format:
- Ethernet
- 802.11 wireless networks
- Bluetooth
- IEEE 802.5 token ring
- most other IEEE 802 networks
- FDDI
- ATM (switched virtual connections only, as part of an NSAP address)
- Fibre Channel and Serial Attached SCSI (as part of a World Wide Name)
- The ITU-T G.hn standard, which provides a way to create a high-speed (up to 1 Gigabit/s) local area network using existing home wiring (power lines, phone lines and coaxial cables). The G.hn Application Protocol Convergence (APC) layer accepts Ethernet frames that use the MAC-48 format and encapsulates them into G.hn Medium Access Control Service Data Units (MSDUs).
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 proprietary term EUI-48 should be used for this purpose.
EUI-64 identifiers are used in:
- FireWire
- IPv6 (as the least-significant 64 bits of a unicast network address or link-local address when stateless autoconfiguration is used)
- ZigBee / 802.15.4 / 6LoWPAN wireless personal-area networks
- Packets sent to the broadcast address, all one bits, are received by all stations on a local area network. In hexadecimal the broadcast address would be "FF:FF:FF:FF:FF:FF".A broadcast frame is flooded and arrived at all nodes. It is accepted by all nodes. See a Broadcasting Simulation video.
- Packets sent to a multicast address are received by all stations on a LAN that have been configured to receive packets sent to that address.That is, a multicast frame is accepted by nodes that have the same multicast address. See a Multicast MAC Simulation video.
- Functional addresses identify one or 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 that could be confused with these forms. The IEEE policy is to discourage new uses of 48-bit identifiers in favor of the EUI-64 system.
IPv6—one of the most prominent standards that uses EUI-64—treats MAC-48 as EUI-48 instead (as it is chosen from the same address pool). This results in extending MAC addresses (such as IEEE 802 MAC address) to EUI-64 using "FF-FE" rather than "FF-FF".
Individual address block
An Individual Address Block is a 24-bit OUI managed by the IEEE Registration Authority, followed by 12 IEEE-provided bits (identifying the organization), and 12 bits for the owner to assign to individual devices. An IAB is ideal for organizations requiring fewer than 4097 unique 48-bit numbers (EUI-48).[5]Bit-reversed notation
The standard notation, also called canonical format, for MAC addresses is written in transmission bit order with the least significant bit transmitted first, as seen in the output of the iproute2/ifconfig/ipconfig command, for example.However, since IEEE 802.3 (Ethernet) and IEEE 802.4 (Token Bus) send the bytes (octets) over the wire, left-to-right, with least significant bit in each byte first, while IEEE 802.5 (Token Ring) and IEEE 802.6 send the bytes over the wire with the most significant bit first, confusion may arise when an address in the latter scenario is represented with bits reversed from the canonical representation. For example, an address in canonical form
12-34-56-78-9A-BC would be transmitted over the wire as bits 01001000 00101100 01101010 00011110 01011001 00111101 in the standard transmission order (least significant bit first). But for Token Ring networks, it would be transmitted as bits 00010010 00110100 01010110 01111000 10011010 10111100 in most significant bit first order. The latter might be incorrectly displayed as 48-2C-6A-1E-59-3D. This is referred to as bit-reversed order, non-canonical form, MSB format, IBM format, or Token Ring format as explained in RFC 2469. Canonical form is generally preferred, and used by all modern implementations.When the first switches which supported both Token Ring and Ethernet came out, many switch vendors didn't understand this canonical form vs non-canonical form difference and as such there were numerous reports all over the world of duplicate MAC addresses. This was actually due to the switches from these vendors, which were not properly reversing the MAC addresses and thus causing the problem.
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