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104-425: MAC addresses are primarily assigned by device manufacturers, and are therefore often referred to as the burned-in address , or as an Ethernet hardware address , hardware address , or physical address . Each address can be stored in the interface hardware, such as its read-only memory , or by a firmware mechanism. Many network interfaces, however, support changing their MAC addresses. The address typically includes

208-602: A binary numeral using the normal mathematical rules for digit significance .” (See hexadecimal ). “The format of a MAC data frame in which the octets of any MAC addresses conveyed in the MAC user data field have the same bit ordering as in the Hexadecimal Representation.” (See MAC data frame , MAC addresses ) This appears from the context of the IEEE Standard 802-2001 to be another term for

312-667: A 36-bit OUI-36 with a 4-bit extension. In either case, the extension was assigned by the organization that purchased the OUI. The resulting identifier was generally represented as a set of octets separated by dashes (hexadecimal notation) or colons (bit-reversed notation) as in FF-FF-FF-FF-FF or FF:FF:FF:FF:FF, as a string of 5 bytes as in {FF,FF,FF,FF,FF}, or as a base 16 number as in FFFFFFFFFF 16 . Note: There were also IAB based CDI-40 sequences that were formed by combining

416-520: A MAC data frame in which the octets of MAC addresses conveyed in the MAC user data field have the same bit ordering as in the Bit-reversed representation.” The order in which an octet or a sequence of octets is transmitted over the transmission medium – this order normally corresponds to the bit-reversed representation. Example: An OUI consisting of the hexadecimal digits ACDE48 16 would be represented as follows: The following figure shows

520-599: A Modified EUI-64 has the following format in significance order: Network Address Authority (NAA) Name_Identifier formats define the first nibble (4 bits) to define the format of the identifier: This encapsulation is used in Fibre Channel and SAS, and is also supported in iSCSI in RFC 3980. This addition requires either a shortened vendor-specific identifier field, or some OUI bits are assumed to be 0, such as when using EUI-64 Mapped format. An Individual Address Block (IAB)

624-632: A PC for firmware updates; for example, a digital audio player might be updated to support a new file format . Some hobbyists have taken advantage of this flexibility to reprogram consumer products for new purposes; for example, the iPodLinux and OpenWrt projects have enabled users to run full-featured Linux distributions on their MP3 players and wireless routers, respectively. ROM is also useful for binary storage of cryptographic data, as it makes them difficult to replace, which may be desirable in order to enhance information security . Since ROM (at least in hard-wired mask form) cannot be modified, it

728-512: A bit line to a word line. Consequently, ROM could be implemented at a lower cost-per- bit than RAM for many years. Most home computers of the 1980s stored a BASIC interpreter or operating system in ROM as other forms of non-volatile storage such as magnetic disk drives were too costly. For example, the Commodore 64 included 64 KB of RAM and 20 KB of ROM containing a BASIC interpreter and

832-524: A company can simply order a large batch of fresh PROM chips and program them with the desired contents at its designers' convenience. The advent of the metal–oxide–semiconductor field-effect transistor (MOSFET), invented at Bell Labs in 1959, enabled the practical use of metal–oxide–semiconductor (MOS) transistors as memory cell storage elements in semiconductor memory , a function previously served by magnetic cores in computer memory . In 1967, Dawon Kahng and Simon Sze of Bell Labs proposed that

936-484: A manufacturer's organizationally unique identifier (OUI). MAC addresses are formed according to the principles of two numbering spaces based on extended unique identifiers (EUIs) managed by the Institute of Electrical and Electronics Engineers (IEEE): EUI-48 —which replaces the obsolete term MAC-48 —and EUI-64 . Network nodes with multiple network interfaces, such as routers and multilayer switches , must have

1040-411: A new role as a medium for mass storage or secondary storage of files. Mask ROM is a read-only memory whose contents are programmed by the integrated circuit manufacturer (rather than by the user). The desired memory contents are furnished by the customer to the device manufacturer. The desired data is converted into a custom photomask /mask layer for the final metallization of interconnections on

1144-511: A specific part of the device, instead of the entire device. This can be done at high speed, hence the name "flash". All of these technologies improved the flexibility of ROM, but at a significant cost-per-chip, so that in large quantities mask ROM would remain an economical choice for many years. (Decreasing cost of reprogrammable devices had almost eliminated the market for mask ROM by the year 2000.) Rewriteable technologies were envisioned as replacements for mask ROM. The most recent development

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1248-452: A specific type of EUI-48 identifier used to address hardware interfaces (e.g., Network Interface Controllers and other network hardware) within existing IEEE 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 – i.e., MAC-48 identifier is identical to the EUI-48 identifier and

1352-553: A string of 8 bytes as in {FF,FF,FF,FF,FF,FF,FF,FF}, or as a base 16 number as in FFFFFFFFFFFFFFFF 16 . Note: According to the IEEE guidelines, the first four digits of the organizationally assigned identifier (i.e., the first four digits of the extension identifier) portion of an EUI-64 “shall not be FFFE 16 or FFFF 16 ” (i.e., EUI-64 identifiers of the form ccccccFFFEeeeeee and ccccccFFFFeeeeee are not allowed) – this

1456-404: A target lifetime of 100 years (until 2080) for applications using EUI-48 space and restricts applications accordingly. The IEEE encourages adoption of the more plentiful EUI-64 for non-Ethernet applications. The distinctions between EUI-48 and MAC-48 identifiers are in name and application only. MAC-48 was used to address hardware interfaces within existing 802-based networking applications; EUI-48

1560-416: A technical level the gains have been achieved by increasing parallelism both in controller design and of storage, the use of large DRAM read/write caches and the implementation of memory cells which can store more than one bit (DLC, TLC and MLC). The latter approach is more failure prone but this has been largely mitigated by overprovisioning (the inclusion of spare capacity in a product which is visible only to

1664-592: A unique MAC address for each network interface in the same network. However, two network interfaces connected to two different networks can share the same MAC address. The IEEE 802 MAC address originally comes from the Xerox Network Systems Ethernet addressing scheme. This 48-bit address space contains potentially 2 (over 281 trillion) possible MAC addresses. The IEEE manages allocation of MAC addresses, originally known as MAC-48 and which it now refers to as EUI-48 identifiers. The IEEE has

1768-406: A variety of other devices. In particular, many microprocessors have mask ROM to store their microcode . Some microcontrollers have mask ROM to store the bootloader or all of their firmware . Classic mask-programmed ROM chips are integrated circuits that physically encode the data to be stored, and thus it is impossible to change their contents after fabrication. It is also possible to write

1872-436: Is NAND flash , also invented at Toshiba. Its designers explicitly broke from past practice, stating plainly that "the aim of NAND flash is to replace hard disks ," rather than the traditional use of ROM as a form of non-volatile primary storage . As of 2021 , NAND has nearly completely achieved this goal by offering throughput higher than hard disks, lower latency, higher tolerance of physical shock, extreme miniaturization (in

1976-475: Is 0 (zero), the frame is meant to reach only one receiving network interface . This type of transmission is called unicast . A unicast frame is transmitted to all nodes within the collision domain . In a modern wired setting (i.e. with switches , not simple hubs ) the collision domain usually is the length of the Ethernet cabling between two network interfaces. In a wireless setting, the collision domain

2080-416: Is a thriving community engaged in the distribution and trading of such software for preservation/sharing purposes. Organizationally unique identifier An organizationally unique identifier ( OUI ) is a 24-bit number that uniquely identifies a vendor , manufacturer , or other organization. OUIs are purchased from the Institute of Electrical and Electronics (IEEE) Registration Authority by

2184-452: Is all receivers that can detect a given wireless signal. If a switch does not know which port leads to a given MAC address, the switch will forward a unicast frame to all of its ports (except the originating port), an action known as unicast flood . Only the node with the matching hardware MAC address will (normally) accept the frame; network interfaces with non-matching MAC-addresses ignore the frame unless they are in promiscuous mode . If

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2288-655: Is an inactive registry activity which has been replaced by the MA-S registry product as of 1 January 2014. The IAB uses a MA-L (and OUI) belonging to the IEEE Registration Authority, concatenated with 12 additional IEEE-provided bits (for a total of 36 bits), leaving only 12 bits for the IAB owner to assign to their (up to 4096) individual devices. An IAB is ideal for organizations requiring not more than 4096 unique 48-bit numbers (EUI-48). Unlike an OUI, which allows

2392-566: Is an inactive registry which has been replaced by the MA-S ( MAC address block, small ), previously named OUI-36 , and has no overlaps in addresses with the IAB registry product as of January 1, 2014. The IAB uses an OUI from the MA-L ( MAC address block, large ) registry, previously called the OUI registry. The term OUI is still in use, but the IEEE Registration Authority does not administer them. An OUI

2496-455: Is an obsolete label for it, although some distinction is still made when encapsulating MAC-48 and EUI-48 identifiers within EUI-64 identifiers (but now, the encapsulating mechanism is also deprecated). The EUI-48 is an identifier that is formed by concatenating the 24-bit OUI with a 24-bit extension identifier that is assigned by the organization that purchased the OUI – the resulting identifier

2600-407: Is common practice to use rewritable non-volatile memory – such as UV- EPROM or EEPROM – for the development phase of a project, and to switch to mask ROM when the code has been finalized. For example, Atmel microcontrollers come in both EEPROM and mask ROM formats. The main advantage of mask ROM is its cost. Per bit, mask ROM was more compact than any other kind of semiconductor memory . Since

2704-408: Is concatenated with 12 additional IEEE-provided bits (for a total of 36 bits), leaving only 12 bits for the organisation owning the IAB to assign to its (up to 4096) individual devices. An IAB is ideal for organizations requiring not more than 4096 unique 48-bit numbers (EUI-48). Unlike an OUI, which allows the assignee to assign values in various different number spaces (for example, EUI-48, EUI-64, and

2808-559: Is generally represented as a set of octets separated by dashes (hexadecimal notation) or colons (bit-reversed notation) as in FF-FF-FF-FF-FF-FF or FF:FF:FF:FF:FF:FF, as a string of 6 bytes as in {FF,FF,FF,FF,FF,FF}, or as a base 16 number as in FFFFFFFFFFFF 16 . The EUI-60 is an identifier that is formed by concatenating the 24-bit OUI with a 36-bit extension identifier that is assigned by the organization that purchased

2912-430: Is now used for 802-based networking and is also used to identify other devices and software, for example Bluetooth . The IEEE now considers MAC-48 to be an obsolete term. EUI-48 is now used in all cases. In addition, the EUI-64 numbering system originally encompassed both MAC-48 and EUI-48 identifiers by a simple translation mechanism. These translations have since been deprecated. The Individual Address Block (IAB)

3016-413: Is only suitable for storing data which is not expected to need modification for the life of the device. To that end, ROM has been used in many computers to store look-up tables for the evaluation of mathematical and logical functions (for example, a floating-point unit might tabulate the sine function in order to facilitate faster computation). This was especially effective when CPUs were slow and ROM

3120-508: Is optional. The following network technologies use the EUI-48 identifier format: Every device that connects to an IEEE 802 network (such as Ethernet and Wi-Fi) has an EUI-48 address. Common networked consumer devices such as PCs, smartphones and tablet computers use EUI-48 addresses. EUI-64 identifiers are used in: On broadcast networks, such as Ethernet, the MAC address is expected to uniquely identify each node on that segment and allows frames to be marked for specific hosts. It thus forms

3224-460: Is rarely changed during the life of the system, also known as firmware . Software applications, such as video games , for programmable devices can be distributed as plug-in cartridges containing ROM . Strictly speaking, read-only memory refers to hard-wired memory, such as diode matrix or a mask ROM integrated circuit (IC), that cannot be electronically changed after manufacture. Although discrete circuits can be altered in principle, through

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3328-410: Is referred to as bit-reversed order , non-canonical form , MSB format , IBM format , or Token Ring format . Read-only memory Read-only memory ( ROM ) is a type of non-volatile memory used in computers and other electronic devices . Data stored in ROM cannot be electronically modified after the manufacture of the memory device . Read-only memory is useful for storing software that

3432-473: Is started with a MAC address set by assigning the last three bytes to be unique on the local network. While this is local administration of MAC addresses, it is not an LAA in the IEEE sense. A historical example of this hybrid situation is the DECnet protocol, where the universal MAC address (OUI AA-00-04, Digital Equipment Corporation) is administered locally. The DECnet software assigns the last three bytes for

3536-415: Is still applied to images of newer games distributed on CD-ROMs or other optical media. ROM images of commercial games, firmware, etc. usually contain copyrighted software. The unauthorized copying and distribution of copyrighted software is a violation of copyright laws in many jurisdictions, although duplication for backup purposes may be considered fair use depending on location. In any case, there

3640-452: Is stored, usually Flash memory ) and RAM. ROM and RAM are essential components of a computer, each serving distinct roles. RAM, or Random Access Memory, is a temporary, volatile storage medium that loses data when the system powers down. In contrast, ROM, being non-volatile, preserves its data even after the computer is switched off. IBM used capacitor read-only storage (CROS) and transformer read-only storage (TROS) to store microcode for

3744-469: Is to support the encapsulation of EUI-48 (FFFE 16 ) and MAC-48 (FFFF 16 ) values into EUI-64 values (though now the encapsulation is deprecated). IPv6 uses a 64-bit Modified Extended Unique Identifier ( Modified EUI-64 ) in the lower half of some IPv6 addresses. A Modified EUI-64 is an EUI-64 with the U/L bit inverted. There are other identifiers that may be formed using the OUI but those listed above are

3848-541: Is typically only possible a certain number of times. The term "ROM" is sometimes used to refer to a ROM device containing specific software or a file with software to be stored in a writable ROM device. For example, users modifying or replacing the Android operating system describe files containing a modified or replacement operating system as " custom ROMs " after the type of storage the file used to be written to, and they may distinguish between ROM (where software and data

3952-409: Is universally or locally administered (e.g., Universal/Local [U/L] bit), etc., and should not contain the values 1, 2, 3, 5, 6, 7, 9, a, b, d, e, or f, unless these values reflect the true meaning of these flag bits – if the organization that owns the OUI does set one of these bits when creating an identifier, then the value of the second nibble of the first octet changes accordingly in representations of

4056-511: Is used to store the basic bootstrapping firmware for the processor, as well as the various firmware needed to internally control self-contained devices such as graphic cards , hard disk drives , solid-state drives , optical disc drives , TFT screens , etc., in the system. Today, many of these "read-only" memories – especially the BIOS / UEFI – are often replaced with EEPROM or Flash memory (see below), to permit in-place reprogramming should

4160-442: Is written in transmission order with the least significant bit of each byte transmitted first, and is used in the output of the ifconfig , ip address , and ipconfig commands, 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 the least significant bit in each byte first, while IEEE 802.5 (Token Ring) and IEEE 802.6 (FDDI) send

4264-651: The Electrotechnical Laboratory in 1972, went a long way to solving problem 4, since an EEPROM can be programmed in-place if the containing device provides a means to receive the program contents from an external source (for example, a personal computer via a serial cable ). Flash memory , invented by Fujio Masuoka at Toshiba in the early 1980s and commercialized in the late 1980s, is a form of EEPROM that makes very efficient use of chip area and can be erased and reprogrammed thousands of times without damage. It permits erasure and programming of only

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4368-632: The KERNAL operating system. Later home or office computers such as the IBM PC XT often included magnetic disk drives, and larger amounts of RAM, allowing them to load their operating systems from disk into RAM, with only a minimal hardware initialization core and bootloader remaining in ROM (known as the BIOS in IBM-compatible computers). This arrangement allowed for a more complex and easily upgradeable operating system. In modern PCs, "ROM"

4472-451: The assignee (IEEE term for the vendor, manufacturer, or other organization). Only assignment from MA-L registry assigns new OUI. They are used to uniquely identify a particular piece of equipments through derived identifiers such as MAC addresses , Subnetwork Access Protocol protocol identifiers, World Wide Names for Fibre Channel devices or vendor blocks in EDID . In MAC addresses,

4576-512: The floating gate of a MOS semiconductor device could be used for the cell of a reprogrammable ROM, which led to Dov Frohman of Intel inventing erasable programmable read-only memory (EPROM) in 1971. The 1971 invention of EPROM essentially solved problem 3, since EPROM (unlike PROM) can be repeatedly reset to its unprogrammed state by exposure to strong ultraviolet light. Electrically erasable programmable read-only memory (EEPROM), developed by Yasuo Tarui, Yutaka Hayashi and Kiyoko Naga at

4680-650: The "locally administered" bit described above. Using wireless access points in SSID -hidden mode ( network cloaking ), a mobile wireless device may not only disclose its own MAC address when traveling, but even the MAC addresses associated to SSIDs the device has already connected to, if they are configured to send these as part of probe request packets. Alternative modes to prevent this include configuring access points to be either in beacon-broadcasting mode or probe-response with SSID mode. In these modes, probe requests may be unnecessary or sent in broadcast mode without disclosing

4784-464: The 'Hexadecimal Representation' – i.e., “by interpreting the bits of the octet value as a binary numeral using the normal mathematical rules for digit significance.” “The representation of a sequence of octet values in which the values of the individual octets are displayed in order from left to right, with each octet value represented as a two-digit hexadecimal numeral, and with the resulting pairs of hexadecimal digits separated by colons . The order of

4888-418: The 1960s, both ROM and its mutable counterpart static RAM were implemented as arrays of transistors in silicon chips; however, a ROM memory cell could be implemented using fewer transistors than an SRAM memory cell, since the latter needs a latch (comprising 5-20 transistors) to retain its contents, while a ROM cell might consist of the absence (logical 0) or presence (logical 1) of one transistor connecting

4992-468: The 36-bit IEEE assigned IAB base value with the 4-bit extension identifier assigned by the organization – e.g., if the IEEE assigned IAB base value is 0x0050C257A and the 4-bit extension identifier is 0xF, then the CDI-40 values generated by combining these two numbers are from 0x0050C257AF00 to 0x0050C257AFFF The IEEE now considers the label MAC-48 to be an obsolete term which was previously used to refer to

5096-664: The EUI-60 was previously used. Some of the storage systems in which an OUI based variant was used are Fibre Channel , and Serial Attached SCSI (SAS). The EUI-64 is an identifier that is formed by concatenating the 24-bit OUI with a 40-bit extension identifier that is assigned by the organization that purchased the OUI – the resulting identifier is generally represented as a set of octets separated by dashes (hexadecimal notation) or colons (bit-reversed notation) as in FF-FF-FF-FF-FF-FF-FF-FF or FF:FF:FF:FF:FF:FF:FF:FF, as

5200-477: The MAC address randomization technique vary largely in different devices. Moreover, various flaws and shortcomings in these implementations may allow an attacker to track a device even if its MAC address is changed, for instance its probe requests' other elements, or their timing. If random MAC addresses are not used, researchers have confirmed that it is possible to link a real identity to a particular wireless MAC address. Randomized MAC addresses can be identified by

5304-408: The OUI (e.g., if the hexadecimal value of the second nibble of the first octet is 'C' and the least-significant-bit is set, then the value becomes 'D'). Notes: The CDI-32 was historically recommended as context dependent identifier that was formed by concatenating the 24-bit OUI with an 8-bit extension identifier that is assigned by the organization that purchased the OUI – the resulting identifier

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5408-485: The OUI is combined with a 24-bit number (assigned by the assignee of the OUI) to form the address. The first three octets of the address are the OUI. The following terms are defined (either implicitly or explicitly) in IEEE Standard 802-2001 for use in referring to the various representations and formats of OUIs and the identifiers that may be created using them. “The representation of a sequence of octet values in which

5512-407: The OUI of AC-DE-48 and that the organization has created the EUI-48 value of AC-DE-48-23-45-67 by concatenating the extension identifier 23-45-67, this EUI-48 identifier has the following format in significance order: The same EUI-48 identifier after encapsulation within an EUI-64 has the following format in significance order: Encapsulation of MAC-48 or EUI-48 within modified EUI-64 example: In

5616-574: The OUI of AC-DE-48 and that the organization has created the MAC-48 value of AC-DE-48-23-45-67 by concatenating the extension identifier 23-45-67, this MAC-48 identifier has the following binary transmission order: The same MAC-48 identifier after encapsulation within an EUI-64 has the following transmission order: The same MAC-48 identifier after encapsulation within an EUI-64 has the following significance order: Encapsulation of EUI-48 within EUI-64 example: Assuming that an organization has registered

5720-399: The OUI value 00:50:C2 was used for IAB assignments. After September 2012, the value 40:D8:55 was used. The owners of an already assigned IAB may continue to use the assignment. The OUI-36 is a deprecated registry activity name, which has been replaced by the MA-S registry product name as of 1 January 2014. This registry activity includes both a 36-bit unique number used in some standards and

5824-514: The OUI – the resulting identifier is generally represented by a string of 15 nibbles, as a base 16 number as in FFFFFFFFFFFFFFF 16 , or as FF-FF-FF:F.F.F.F.F.F.F.F.F as an EUI-64 value. Note: This identifier was previously used as the worldwide name (WWN) identifier within some storage systems. Its use is now considered deprecated by the IEEE and the EUI-64 identifier should be used in the future for this and all other purposes for which

5928-466: The ROM chips, and the presence or absence of these features will represent either a 1 or a 0 bit, depending on the ROM design. Thus by design, any attempts to electronically change the data will fail, since the data is defined by the presence or absence of physical features or structures that cannot be electronically changed. For every software program, even for revisions of the same program, the entire mask must be changed, which can be costly. In mask ROM,

6032-569: The US National Security Agency has a system that tracks the movements of mobile devices in a city by monitoring MAC addresses. To avert this practice, Apple has started using random MAC addresses in iOS devices while scanning for networks. Other vendors followed quickly. MAC address randomization during scanning was added in Android starting from version 6.0, Windows 10, and Linux kernel 3.18. The actual implementations of

6136-546: The addition of bodge wires and the removal or replacement of components, ICs cannot. Correction of errors, or updates to the software, require new devices to be manufactured and to replace the installed device. Floating-gate ROM semiconductor memory in the form of erasable programmable read-only memory (EPROM), electrically erasable programmable read-only memory (EEPROM) and flash memory can be erased and re-programmed. But usually, this can only be done at relatively slow speeds, may require special equipment to achieve, and

6240-574: The assignee to assign values in various different number spaces (for example, EUI-48, EUI-64, and the various context-dependent identifier number spaces), the Individual Address Block could only be used to assign EUI-48 identifiers. All other potential uses based on the OUI from which the IABs are allocated are reserved, and remain the property of the IEEE Registration Authority. It should also be noted that, between 2007 and September 2012,

6344-600: The assignment of a MAC address to network interface when scanning for wireless access points to avert tracking systems. In Internet Protocol (IP) networks, the MAC address of an interface corresponding to an IP address may be queried with the Address Resolution Protocol (ARP) for IPv4 and the Neighbor Discovery Protocol (NDP) for IPv6, relating OSI layer 3 addresses with layer 2 addresses. According to Edward Snowden ,

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6448-403: The assignment of a block of EUI-48 and EUI-64 identifiers (while owner of IAB cannot assign EUI-64) by the IEEE Registration Authority. The owner of an already assigned OUI-36 registry product may continue to use the assignment. Example of EUI-48 created within an IAB: An EUI-48 identifier is formed by combining the 36-bit IEEE assigned IAB base value with a 12-bit extension identifier assigned by

6552-481: The attached addresses to activate. Hence, various configuration scripts and utilities permit the randomization of the MAC address at the time of booting or before establishing a network connection. Changing MAC addresses is necessary in network virtualization . In MAC spoofing , this is practiced in exploiting security vulnerabilities of a computer system. Some modern operating systems, such as Apple iOS and Android, especially in mobile devices, are designed to randomize

6656-408: The basis of most of the link layer (OSI layer 2 ) networking upon which upper-layer protocols rely to produce complex, functioning networks. Many network interfaces support changing their MAC address. On most Unix -like systems, the command utility ifconfig may be used to remove and add link address aliases. For instance, the active ifconfig directive may be used on NetBSD to specify which of

6760-426: The bytes over the wire with the most significant bit first, confusion may arise when an OUI in the latter scenario is represented with bits reversed from the canonical representation. So for instance, an OUI whose canonical form is ACDE48 could be seen written as 357B12 if translation is done improperly or inconsistently. The latter form ( bit-reversed or noncanonical representation), may also be referred to in

6864-650: 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

6968-518: The complete MAC address to be AA-00-04-00-XX-YY where XX-YY reflects the DECnet network address xx.yy of the host. This eliminates the need for DECnet to have an address resolution protocol since the MAC address for any DECnet host can be determined from its DECnet address. The least significant bit of an address's first octet is referred to as the I/G , or Individual/Group , bit. When this bit

7072-423: The contents of a Laser ROM by using a laser to alter the electrical properties of only some diodes on the ROM, or by using a laser to cut only some polysilicon links, instead of using a mask. By applying write protection , some types of reprogrammable ROMs may temporarily become read-only memory. There are other types of non-volatile memory which are not based on solid-state IC technology, including: Although

7176-660: The cost of an integrated circuit strongly depends on its size, mask ROM is significantly cheaper than any other kind of semiconductor memory. However, the one-time masking cost is high and there is a long turn-around time from design to product phase. Design errors are costly: if an error in the data or code is found, the mask ROM is useless and must be replaced in order to change the code or data. As of 2003 , four companies produce most such mask ROM chips: Samsung Electronics , NEC Corporation , Oki Electric Industry , and Macronix . Some integrated circuits contain only mask ROM. Other integrated circuits contain mask ROM as well as

7280-463: The data is physically encoded in the circuit, so it can only be programmed during fabrication. This leads to a number of serious disadvantages: Subsequent developments have addressed these shortcomings. Programmable read-only memory (PROM), invented by Wen Tsing Chow in 1956, allowed users to program its contents exactly once by physically altering its structure with the application of high-voltage pulses. This addressed problems 1 and 2 above, since

7384-453: The drive controller) and by increasingly sophisticated read/write algorithms in drive firmware. Because they are written by forcing electrons through a layer of electrical insulation onto a floating transistor gate , rewriteable ROMs can withstand only a limited number of write and erase cycles before the insulation is permanently damaged. In the earliest EPROMs, this might occur after as few as 1,000 write cycles, while in modern Flash EEPROM

7488-455: The encapsulation within a Modified EUI-64 a MAC-48 is treated as an EUI-48 and the U/L bit is inverted. Assuming that an organization has registered the OUI of AC-DE-48 and that the organization has created the MAC-48 or EUI-48 value of AC-DE-48-23-45-67 by concatenating the extension identifier 23-45-67, this MAC-48 or EUI-48 identifier has the following format in significance order: The same MAC-48 or EUI-48 identifier after encapsulation within

7592-418: The endurance may exceed 1,000,000. The limited endurance, as well as the higher cost per bit, means that Flash-based storage is unlikely to completely supplant magnetic disk drives in the near future. The timespan over which a ROM remains accurately readable is not limited by write cycling. The data retention of EPROM, EAROM, EEPROM, and Flash may be time-limited by charge leaking from the floating gates of

7696-501: The first 28 bits being assigned by IEEE. The first 24 bits of the assigned MA-M block are an OUI assigned to IEEE that will not be reassigned, so the MA-M does not include assignment of an OUI. Addresses can either be universally administered addresses (UAA) or locally administered addresses (LAA). A universally administered address is uniquely assigned to a device by its manufacturer. The first three octets (in transmission order) identify

7800-483: The first octet is 06 (hexadecimal), the binary form of which is 000001 1 0, where the second-least-significant bit is 1. Therefore, it is a locally administered address. Even though many hypervisors manage dynamic MAC addresses within their own OUI , often it is useful to create an entire unique MAC within the LAA range. In virtualisation , hypervisors such as QEMU and Xen have their own OUIs. Each new virtual machine

7904-404: The first octet of the hexadecimal representation (i.e., the two least significant bits of the first octet) of the OUI are reserved as flag bits for some protocols (e.g., 'M' bit and 'X' bit), flags to indicate whether the address is part of an individual (unicast) or group (multicast) address block (e.g., Individual/Group [I/G] bit or Unicast/Multicast [U/M] bit), flags to indicate whether an address

8008-721: The form of USB flash drives and tiny microSD memory cards , for example), and much lower power consumption. Many stored-program computers use a form of non-volatile storage (that is, storage that retains its data when power is removed) to store the initial program that runs when the computer is powered on or otherwise begins execution (a process known as bootstrapping , often abbreviated to " booting " or "booting up"). Likewise, every non-trivial computer needs some form of mutable memory to record changes in its state as it executes. Forms of read-only memory were employed as non-volatile storage for programs in most early stored-program computers, such as ENIAC after 1948 . (Until then it

8112-489: The hexadecimal digits in each pair, and the mapping between the hexadecimal digits and the bits of the octet value, are derived by reversing the order of the bits in the octet value and interpreting the resulting bit sequence as a binary numeral using the normal mathematical rules for digit significance.” The bit-reversed representation corresponds to the convention of transmitting the least-significant-bit of each byte first in serial data communications . “The format of

8216-650: The identity of previously known networks. The standard ( IEEE 802 ) format for printing EUI-48 addresses in human-friendly form 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 (e.g. 01-23-45-67-89-AB-CD-EF ). Other conventions include six groups of two hexadecimal digits separated by colons (:) (e.g. 01:23:45:67:89:AB ), and three groups of four hexadecimal digits separated by dots (.) (e.g. 0123.4567.89AB ); again in transmission order. The standard notation, also called canonical format, for MAC addresses

8320-442: The integrated circuit, although fuse re-growth was once a problem in some systems. The contents of ROM chips can be extracted with special hardware devices and relevant controlling software. This practice is common for, as a main example, reading the contents of older video game console cartridges . Another example is making backups of firmware/OS ROMs from older computers or other devices - for archival purposes, as in many cases,

8424-499: The invention of the integrated circuit came mask ROM . Mask ROM consists of a grid of word lines (the address input) and bit lines (the data output), selectively joined with transistor switches, and can represent an arbitrary look-up table with a regular physical layout and predictable propagation delay . Mask ROM is programmed with photomasks in photolithography during semiconductor manufacturing . The mask defines physical features or structures that will be removed, or added in

8528-483: The larger identifier – this process is known as 'encapsulation' and is provided for the purpose of easing the transition from MAC-48 and EUI-48 to EUI-64 and to provide a mechanism for the conversion of MAC-48 and EUI-48 identifiers to EUI-64 in such a way that duplicate or conflicting values are avoided. Encapsulation of MAC-48 within EUI-64 Example: Assuming that an organization has registered

8632-624: The least significant bit of the first octet is set to 1 (i.e. the second hexadecimal digit is odd) the frame will still be sent only once; however, network interface controllers will choose to accept or ignore it based on criteria other than the matching of their individual MAC addresses: for example, based on a configurable list of accepted multicast MAC addresses. This is called multicast addressing. The IEEE has built in several special address types to allow more than one network interface card to be addressed at one time: These are all examples of group addresses , as opposed to individual addresses ;

8736-471: The least significant bit of the first octet of a MAC address distinguishes individual addresses from group addresses. That bit is set to 0 in individual addresses and set to 1 in group addresses. Group addresses, like individual addresses, can be universally administered or locally administered. The U/L and I/G bits are handled independently, and there are instances of all four possibilities. IPv6 multicast uses locally administered, multicast MAC addresses in

8840-405: The literature as "MSB format", "IBM format", or "Token Ring format" for this reason. RFC2469 explains the problem in more detail. The OUI is normally discussed and represented as a set of octets in hexadecimal notation separated by dashes (i.e., FF-FF-FF) or as a set of octets separated by colons in bit-reversed notation (i.e., FF:FF:FF). The two least-significant-bits of the second nibble of

8944-475: The memory cell transistors. Early generation EEPROM's, in the mid-1980s generally cited 5 or 6 year data retention. A review of EEPROM's offered in the year 2020 shows manufacturers citing 100 year data retention. Adverse environments will reduce the retention time (leakage is accelerated by high temperatures or radiation ). Masked ROM and fuse/antifuse PROM do not suffer from this effect, as their data retention depends on physical rather than electrical permanence of

9048-548: The memory chip (hence the name). Mask ROM can be made in several ways, all of which aim to change the electrical response of a transistor when it is addressed on a grid, such as: Mask ROM transistors can be arranged in either NOR or NAND configurations and can achieve one of the smallest cell sizes possible as each bit is represented by only one transistor. NAND offers higher storage density than NOR. OR configurations are also possible, but compared to NOR it only connects transistors to V cc instead of V ss . Mask ROMs used to be

9152-407: The most commonly used. Mapping an EUI-48 to an EUI-64 is deprecated. The mapping is described here for historical reasons. Other identifiers, such as MAC-48 and EUI-48 values, can be contained within a larger identifier or 'container', such as EUI-64, by creating the larger identifier through a process of combining the smaller identifier with specified values placed in specified bit-positions within

9256-417: The most inexpensive, and are the simplest semiconductor memory devices, with only one metal layer and one polysilicon layer, making it the type of semiconductor memory with the highest manufacturing yield (the highest number of working devices per manufacturing run). ROM can be made using one of several semiconductor device fabrication technologies such as CMOS , nMOS , pMOS , and bipolar transistors . It

9360-405: The movement of jumper plugs to apply write-enable signals, and special lock/unlock command codes. Modern NAND Flash can be used to achieve the highest write speeds of any rewritable ROM technology, with speeds as high as 10 GB / s in an SSD. This has been enabled by the increased investment in both consumer and enterprise solid-state drives and flash memory products for higher end mobile devices. On

9464-858: The need for a firmware upgrade arise. However, simple and mature sub-systems (such as the keyboard or some communication controllers in the integrated circuits on the main board, for example) may employ mask ROM or OTP (one-time programmable). ROM and successor technologies such as flash are prevalent in embedded systems . These are in everything from industrial robots to home appliances and consumer electronics ( MP3 players , set-top boxes , etc.) all of which are designed for specific functions, but are based on general-purpose microprocessors . With software usually tightly coupled to hardware, program changes are rarely needed in such devices (which typically lack hard disks for reasons of cost, size, or power consumption). As of 2008, most products use Flash rather than mask ROM, and many provide some means for connecting to

9568-554: The organization that issued the identifier and are known as the organizationally unique identifier (OUI). The remainder of the address (three octets for EUI-48 or five for EUI-64) are assigned by that organization in nearly any manner they please, subject to the constraint of uniqueness. A locally administered address is assigned to a device by software or a network administrator, overriding the burned-in address for physical devices. Locally administered addresses are distinguished from universally administered addresses by setting (assigning

9672-459: The original chips are PROMs and thus at risk of exceeding their usable data lifetime. The resultant memory dump files are known as ROM images or abbreviated ROMs , and can be used to produce duplicate ROMs - for example to produce new cartridges or as digital files for playing in console emulators . The term ROM image originated when most console games were distributed on cartridges containing ROM chips, but achieved such widespread usage that it

9776-442: The position of these bits in significance order: Notes: Ethernet users are used to seeing canonical form , such as in the output of the ifconfig command. Canonical form is the intended standard. 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 ( FDDI ) send

9880-401: The range 3 3 -33-XX-XX-XX-XX (with both bits set). Given the locations of the U/L and I/G bits, they can be discerned in a single digit in common MAC address notation as shown in the following table: IEEE standard 802c further divides the locally administered MAC address block into four quadrants. This additional partitioning is called Structured Local Address Plan (SLAP) and its usage

9984-443: The relative speed of RAM vs. ROM has varied over time, as of 2007 large RAM chips can be read faster than most ROMs. For this reason (and to allow uniform access), ROM content is sometimes copied to RAM or shadowed before its first use, and subsequently read from RAM. For those types of ROM that can be electrically modified, writing speed has traditionally been much slower than reading speed, and it may need unusually high voltage,

10088-583: The smaller System/360 models, the 360/85 , and the initial two System/370 models ( 370/155 and 370/165 ). On some models there was also a writeable control store (WCS) for additional diagnostics and emulation support. The Apollo Guidance Computer used core rope memory , programmed by threading wires through magnetic cores. The simplest type of solid-state ROM is as old as the semiconductor technology itself. Combinational logic gates can be joined manually to map n -bit address input onto arbitrary values of m -bit data output (a look-up table ). With

10192-519: The value 40:D8:55 was used. The owners of an already assigned IAB may continue to use the assignment. The MA-S registry includes, for each registrant, both a 36-bit unique number used in some standards and a block of EUI-48 and EUI-64 identifiers (while the registrant of an IAB cannot assign an EUI-64). MA-S does not include assignment of an OUI. Additionally, the MA-M ( MAC address block, medium ) provides both 2 EUI-48 identifiers and 2 EUI-64 identifiers,

10296-452: The value of 1 to) the second- least-significant bit of the first octet of the address. This bit is also referred to as the U/L bit, short for Universal/Local , which identifies how the address is administered. If the bit is 0, the address is universally administered, which is why this bit is 0 in all UAAs. If it is 1, the address is locally administered. In the example address 06-00-00-00-00-00

10400-416: The values of the individual octets are displayed in order from left to right, with each octet value represented as a two-digit hexadecimal numeral, and with the resulting pairs of hexadecimal digits separated by hyphens . The order of the hexadecimal digits in each pair, and the mapping between the hexadecimal digits and the bits of the octet value, are derived by interpreting the bits of the octet value as

10504-418: The various context-dependent identifier number spaces, like for SNAP or EDID ), the Individual Address Block could only be used to assign EUI-48 identifiers. All other potential uses based on the OUI from which the IABs are allocated are reserved and remain the property of the IEEE Registration Authority. Between 2007 and September 2012, the OUI value 00:50:C2 was used for IAB assignments. After September 2012,

10608-575: Was cheap compared to RAM. Notably, the display adapters of early personal computers stored tables of bitmapped font characters in ROM. This usually meant that the text display font could not be changed interactively. This was the case for both the CGA and MDA adapters available with the IBM PC XT. The use of ROM to store such small amounts of data has disappeared almost completely in modern general-purpose computers. However, NAND Flash has taken over

10712-527: Was generally represented as a set of octets separated by dashes (hexadecimal notation) or colons (bit-reversed notation) as in FF-FF-FF-FF or FF:FF:FF:FF, as a string of 4 bytes as in {FF,FF,FF,FF}, or as a base 16 number as in FFFFFFFF 16 . The CDI-40 was historically recommended as context dependent identifier that was formed by concatenating the 24-bit OUI with a 16-bit extension or by concatenating

10816-408: Was not a stored-program computer as every program had to be manually wired into the machine, which could take days to weeks.) Read-only memory was simpler to implement since it needed only a mechanism to read stored values, and not to change them in-place, and thus could be implemented with very crude electromechanical devices (see historical examples below). With the advent of integrated circuits in

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