View source for Ethernet

{{Short description|Computer networking technology}}
{{Use mdy dates|date=March 2023}}
{{Use American English|date=April 2023}}

[[File:Apple Ethernet Symbol.svg|thumb|100px|Symbol used by [[Apple Inc.|Apple]] and [[Google]] on some devices to denote an Ethernet connection]]

'''Ethernet''' ({{IPAc-en|ˈ|iː|θ|ər|n|ɛ|t}} {{respell|EE|thər|net}}) is a family of wired [[computer network]]ing technologies commonly used in [[local area network]]s (LAN), [[metropolitan area network]]s (MAN) and [[wide area network]]s (WAN).<ref>{{cite web | url = https://www.mef.net/Assets/White_Papers/Metro-Ethernet-Services.pdf | title = Metro Ethernet Services – A Technical Overview | year = 2003 | access-date = January 9, 2016 | author = Ralph Santitoro | website = mef.net | archive-date = December 22, 2018 | archive-url = https://web.archive.org/web/20181222184046/http://www.mef.net/Assets/White_Papers/Metro-Ethernet-Services.pdf | url-status = dead }}</ref> It was commercially introduced in 1980 and first standardized in 1983 as [[IEEE 802.3]]. Ethernet has since been refined to support higher [[bit rate]]s, a greater number of nodes, and longer link distances, but retains much [[backward compatibility]]. Over time, Ethernet has largely replaced competing wired LAN technologies such as [[Token Ring]], [[FDDI]] and [[ARCNET]].

The original [[10BASE5]] Ethernet uses a thick [[coaxial cable]] as a [[shared medium]]. This was largely superseded by [[10BASE2]], which used a thinner and more flexible cable that was both less expensive and easier to use. More modern Ethernet variants use [[Ethernet over twisted pair|twisted pair]] and [[fiber optic]] links in conjunction with [[Network switch|switches]]. Over the course of its history, Ethernet data transfer rates have been increased from the original {{val|2.94|ul=Mbit/s}}<ref name="Alto">{{cite web|last1=Xerox|title=Alto: A Personal Computer System Hardware Manual|url=http://www.bitsavers.org/pdf/xerox/alto/Alto_Hardware_Manual_Aug76.pdf|publisher=Xerox|date=August 1976|access-date=August 25, 2015|page=37|archive-date=September 4, 2017|archive-url=https://web.archive.org/web/20170904111228/http://bitsavers.org/pdf/xerox/alto/Alto_Hardware_Manual_Aug76.pdf|url-status=live}}</ref> to the latest [[Terabit Ethernet|{{val|800|u=Gbit/s}}]], with rates up to {{val|1.6|ul=Tbit/s}} under development. The [[:Category:Ethernet standards|Ethernet standards]] include several [[Ethernet physical layer|wiring and signaling variants]] of the [[Physical layer|OSI physical layer]].

Systems communicating over Ethernet divide a stream of data into shorter pieces called [[Frame (networking)|frames]]. Each frame contains source and destination addresses, and [[frame check sequence|error-checking data]] so that damaged frames can be detected and discarded; most often, higher-layer protocols trigger [[retransmission (data networks)|retransmission]] of lost frames. Per the [[OSI model]], Ethernet provides services up to and including the [[data link layer]].<ref>{{cite web| url = http://www.tcpipguide.com/free/t_DataLinkLayerLayer2.htm| title = Data Link Layer (Layer&nbsp;2)| date = September 20, 2005| access-date = January 9, 2016| author = Charles M. Kozierok| website = tcpipguide.com| archive-date = May 20, 2019| archive-url = https://web.archive.org/web/20190520101511/http://www.tcpipguide.com/free/t_DataLinkLayerLayer2.htm| url-status = live}}</ref> The 48-bit [[MAC address]] was adopted by other [[IEEE 802]] networking standards, including [[IEEE 802.11]] ([[Wi-Fi]]), as well as by [[FDDI]]. [[EtherType]] values are also used in [[Subnetwork Access Protocol]] (SNAP) headers.

Ethernet is widely used in homes and industry, and interworks well with wireless [[Wi-Fi]] technologies. The [[Internet Protocol]] is commonly carried over Ethernet and so it is considered one of the key technologies that make up the [[Internet]].
==History==
[[File:Accton-etherpocket-sp-parallel-port-ethernet-adapter.jpg|thumb|[[Accton Technology Corporation|Accton]] Etherpocket-SP [[parallel port]] Ethernet adapter ({{Circa|1990}}). Supports both coaxial ([[10BASE2]]) and twisted pair ([[10BASE-T]]) cables. Power is drawn from a [[PS/2 port]] passthrough cable.]]

The original forms of Ethernet used a shared communications channel. This concept originated in [[ALOHAnet]], designed in the late 1960s by [[Norman Abramson]]. ALOHANet was a 4800 bps radio network used by the University of Hawaii. When a sender detected that its message hadn't been received, it would resend the message after waiting for a randomly selected period of time.<ref name=Breyer>{{cite book |last=Breyer |first=Robert |date=1999 |title=Switched, fast, and gigabit Ethernet|url=https://archive.org/details/switchedfastgiga0003brey/|location=U.S.A. |publisher=MacMillan Technical Publications|isbn=9781578700738 |access-date=November 7, 2025}}</ref>{{rp|3-4}}  

In 1972, [[Robert Metcalfe]] and [[David Boggs]] adapted the ALOHAnet approach to transmission over a shared coaxial cable in the [[Xerox Palo Alto Research Center]] (Xerox PARC). This network connected ALTO computers using a coaxial cable. It first ran on May 22, 1973 with a bit rate of 2.94 Mbps. In a memo written at that time, Metcalfe named the concept “Ethernet.” The name was inspired by the former idea that the universe was filled with a "[[luminiferous aether]]" that carried electromagnetic waves, and calling it Ethernet emphasized its ability to run over any transmission medium.<ref>{{cite web | title=Exclusive Interview - Bob Metcalfe the Father of Ethernet &#124; | date=August 25, 2017 | url=https://broadbandlibrary.com/bob-metcalfe-lays-down-the-law/ }}</ref> Ethernet improved the original ALOHANet design because a sender would first listen to the channel to determine if it was already in use. The combination of the new idea of ''Carrier Sense'' with ''Multiple Access'' and ''Collision Detection'' from ALOHANet became Carrier-Sense Multiple Access/Collision Detection, or [[CSMA/CD]].<ref name=Breyer/>{{rp|6-7}}

In 1975, Metcalfe, Boggs and their colleagues [[Charles P. Thacker|Charles Thacker]] and [[Butler Lampson]] filed for a patent on Ethernet, which was granted in 1977.<ref>{{US patent|4063220}} "Multipoint data communication system (with collision detection)"</ref> By 1976, 100 ALTOs at Xerox PARC were connected using Ethernet. In July 1976, Metcalfe and Boggs published the seminal paper ''Ethernet: Distributed Packet Switching for Local Computer Networks'' in [[Communications of the ACM]] (CACM).<ref name=Breyer/>{{rp|7}}<ref>{{cite journal| author1 = Robert Metcalfe| author2 = David Boggs| date = July 1976| title = Ethernet: Distributed Packet Switching for Local Computer Networks| journal = [[Communications of the ACM]]| volume = 19| issue = 7| pages = 395–405| doi = 10.1145/360248.360253| url = https://dl.acm.org/doi/pdf/10.1145/360248.360253| s2cid = 429216| author-link1 = Robert Metcalfe| author-link2 = David Boggs| access-date = August 25, 2015| archive-date = March 15, 2016| archive-url = https://web.archive.org/web/20160315040642/http://research.microsoft.com/en-us/um/people/pcosta/cn_slides/metcalfe76ethernet.pdf| url-status = live}}</ref> Also in 1976, [[Ron Crane (engineer)|Ron Crane]], [[Bob Garner]], and [[Roy Ogus]] designed a 10Mbps version of Ethernet running over coaxial cable.<ref>{{Cite web|url=https://www.wband.com/2013/05/introduction-to-ethernet-technologies/|title=Introduction to Ethernet Technologies|publisher=WideBand Products|website=www.wband.com|language=en-US|access-date=April 9, 2018|archive-date=April 10, 2018|archive-url=https://web.archive.org/web/20180410072256/https://www.wband.com/2013/05/introduction-to-ethernet-technologies/|url-status=live}}</ref> 

There were multiple local area network technologies in the 1970s. These included IBM's [[token ring]], Network Systems Corporation's [[HYPERchannel]] and Datapoint's [[ARCnet]]. All were proprietary at the time. Metcalfe developed a strategy of standardizing Ethernet rather than keeping it vendor-specific, and convinced [[Digital Equipment Corporation]] (DEC), [[Intel]], and [[Xerox]] to work together on a standard, subsequently known as the DIX standard, based on the 10Mbps version of Ethernet and published in 1980 as the Ethernet Blue Book.<ref>{{cite journal |author1=Digital Equipment Corporation |author2=Intel Corporation |author3=Xerox Corporation |title=The ethernet: a local area network: data link layer and physical layer specifications |journal=ACM SIGCOMM Computer Communication Review |url=https://dl.acm.org/doi/10.1145/1015591.1015594|date=July 1981 |volume=11 |issue=3 |pages=20–66 |doi=10.1145/1015591.1015594 |url-access=subscription }}</ref> Version 2 was published in November 1982.<ref>{{Cite report |url=http://decnet.ipv7.net/docs/dundas/aa-k759b-tk.pdf |title=The Ethernet, A Local Area Network. Data Link Layer and Physical Layer Specifications, Version 2.0 |date=November 1982 |author1=Digital Equipment Corporation |author2=Intel Corporation |author3=Xerox Corporation |publisher=Xerox Corporation |access-date=December 10, 2011 |archive-date=December 15, 2011 |archive-url=https://web.archive.org/web/20111215224455/http://decnet.ipv7.net/docs/dundas/aa-k759b-tk.pdf |url-status=live }}</ref><ref name=Breyer/>{{rp| 7-8}} 

In June 1981, the [[Institute of Electronic and Electrical Engineers]] (IEEE) Project [[802]] (for local area network standards) created an [[802.3]] subcommittee to produce an Ethernet standard based on DIX.  In 1983, a standard was published for 10 Mbps Ethernet over a coaxial cable of up to 500 meters (10BASE5). It differed only in some details from the DIX standard. As part of the standardization process, Xerox turned over all its Ethernet patents to the IEEE, and anyone can implement 802.3. IEEE 802.3 is now considered the same as Ethernet.<ref name=Breyer/>{{rp|8}}

In June 1979, Metcalfe left Xerox to found the Computer, Communication, and Compatibility Corporation, better known as [[3Com]], along with [[Howard Charney]], [[Ron Crane (engineer)|Ron Crane]], [[Greg Shaw]], and [[Bill Kraus]]. Metcalfe's vision was to sell Ethernet adapters for all personal computers. Apple quickly agreed, but IBM was committed to their own LAN protocol, the Token Ring. Nonetheless, 3Com developed the EtherLink [[Industry Standard Architecture|ISA]] adapter and started shipping it with DOS driver software, making it usable on IBM PCs.<ref name=Breyer/>{{rp|9}}

The EtherLink adapter had several advantages over competitors. It was the first network interface card (NIC) to use VLSI semiconductor technology (developed in partnership with [[Seeq Technologies]]). This meant most of the functions, including the transceiver, could be contained on a single chip, so the price for Etherlink ($950) was significantly lower than of its competitors. 3Com introduced a new, thinner coaxial cable for the card, called [[Thin Ethernet]], making it more convenient to install and use.  Finally, Etherlink was the first Ethernet adapter for the IBM PC.<ref name=Breyer/>{{rp|9-10}}

Because both businesses and home users adopted the IBM PC, its market expanded rapidly, and by 1982, IBM was shipping 200,000 units a month. Since IBM hadn't realized that businesses would want the computers connected by a network, Etherlink sales filled the vacuum and in 1984 3Com was able to file for a public stock offering. The Etherlink approach was standardized by IEEE as 10BASE2 in 1984.<ref name=Breyer/>{{rp|11}}

Also in the early 1980s, [[Novell]] began selling Network Interface Cards (NICs) to go with its NetWare operating system. These NE2000 NICs were all Ethernet, and because NetWare became an important application for businesses, this increased the demand for Ethernet adapters. Then in 1989, Novell sold its NIC business and licensed the NE2000 card, creating a highly competitive market and driving the price of Ethernet cards down, while cards for other technologies such as IBM’s token ring remained high.<ref name=Breyer/>{{rp|16-17}} 

Starting in late 1983, [[AT&T]] and [[NCR Voyix|NCR]] promoted a star configuration using unshielded twisted pair cabling (UTP), or regular telephone wire. This became [[StarLAN]], running at 1Mbps over cables up to 500 meters, and was standardized as 1BASE5 by IEEE 802.3,<ref name=Breyer/>{{rp|12-13}} but on August 17, 1987, [[SynOptics]] introduced LATTISNET with 10Mbps Ethernet also over regular telephone wire (UTP).<ref name=Breyer/>{{rp|14}} In the fall of 1990, the IEEE issued the 802.3i standard for 10BASE-T, Ethernet over twisted pairs, and the following year, Ethernet sales nearly doubled.<ref name=Breyer/>{{rp|15-16}} By 1992, Ethernet was the de facto standard for LANS.<ref name=Breyer/>{{rp|17}}

==Standardization==
[[File:An Intel 82574L Gigabit Ethernet NIC, PCI Express x1 card.jpg|thumb|right|An Intel 82574L Gigabit Ethernet NIC, PCI Express ×1 card]]

In February 1980, the [[Institute of Electrical and Electronics Engineers]] (IEEE) started project [[IEEE 802|802]] to standardize local area networks (LAN).<ref name=VonBurg2003 /><ref>{{cite web |url=http://www.ieeeusa.org/policy/policy/2001/01aug27IEEE802.pdf |title=Letter to FCC |author=Vic Hayes |date=August 27, 2001 |quote=IEEE 802 has the basic charter to develop and maintain networking standards... IEEE 802 was formed in February 1980... |access-date=October 22, 2010 |archive-url=https://web.archive.org/web/20110727094219/http://www.ieeeusa.org/policy/policy/2001/01aug27IEEE802.pdf |archive-date=July 27, 2011 |url-status=dead }}</ref> The DIX group with Gary Robinson (DEC), Phil Arst (Intel), and Bob Printis (Xerox)<ref>{{cite web |last=Robinson |first=Gary |date=2011 |title=IEEE P802 view of history by Gary Robinson based upon the paper “Standardization of Local Area Networks” by Marvin Sirbu of Carnegie Mellon University and Kent Hughes of Pacific Bell|url=http://ieeehc.pairserver.com/pubs/802/sirbu-hughes-notes-by-robinson-only.pdf |website=IEEE |location=United States |publisher=IEEE |access-date=November 11, 2025}}</ref> submitted the so-called ''Blue Book'' [[CSMA/CD]] specification as a candidate for the LAN specification.<ref name="blue">{{Cite report |url=http://ethernethistory.typepad.com/papers/EthernetSpec.pdf |url-status=dead |date=September 30, 1980 |title=The Ethernet, A Local Area Network. Data Link Layer and Physical Layer Specifications, Version 1.0 |author1=Digital Equipment Corporation |author2=Intel Corporation |author3=Xerox Corporation |publisher=Xerox Corporation |access-date=December 10, 2011|archive-date=August 25, 2019 |archive-url=https://web.archive.org/web/20190825014958/https://ethernethistory.typepad.com/papers/EthernetSpec.pdf }}</ref> In addition to CSMA/CD, Token Ring (supported by IBM) and Token Bus (selected and henceforward supported by [[General Motors]]) were also considered as candidates for a LAN standard. Competing proposals and broad interest in the initiative led to strong disagreement over which technology to standardize. In December 1980, the group was split into three subgroups, and standardization proceeded separately for each proposal.<ref name=froehlich>{{cite book |editor-last1=Froehlich |editor-first1=Fritz E. |editor-last2=Kent |editor-first2=Allen |url=https://www.google.com/books/edition/The_Froehlich_Kent_Encyclopedia_of_Telec/lQ6NeDDr4o4C?hl=en&gbpv=1&dq=token+bus+802.3+general+motors&pg=PA2&printsec=frontcover|date=1990 |title=The Froehlich/Kent Encyclopedia of Telecommunications  |volume=9. IEEE 802.3 and Ethernet Standards to Interrelationship of the SS7 Protocol Architecture and the OSI Reference Model and Protocols |location=New York, Basel, Hong Kong |isbn=0-8247-2907-2 |pages=1-2}}</ref><ref name=VonBurg2003>{{cite journal |last1=von Burg |first1=Urs |last2=Kenney |first2=Martin |title=Sponsors, Communities, and Standards: Ethernet vs. Token Ring in the Local Area Networking Business |journal=Industry & Innovation |date=December 2003 |volume=10 |issue=4 |pages=351–375 |doi=10.1080/1366271032000163621 |s2cid=153804163 |url=http://hcd.ucdavis.edu/faculty/webpages/kenney/articles_files/Sponsors,%20Communities,%20and%20Standards:%20Ethernet%20vs.%20Token%20Ring%20in%20the%20Local%20Area%20Networking%20Business.pdf |archive-url=https://web.archive.org/web/20111206202221/http://hcd.ucdavis.edu/faculty/webpages/kenney/articles_files/Sponsors,%20Communities,%20and%20Standards:%20Ethernet%20vs.%20Token%20Ring%20in%20the%20Local%20Area%20Networking%20Business.pdf |archive-date=December 6, 2011 |url-status=dead |access-date=February 17, 2014  }}</ref>

Delays in the standards process put at risk the market introduction of the [[Xerox Star]] workstation and 3Com's Ethernet LAN products. With such business implications in mind, [[David Liddle]] (General Manager, Xerox Office Systems) and Metcalfe (3Com) strongly supported a proposal of Fritz Röscheisen ([[Siemens]] Private Networks) for an alliance in the emerging office communication market, including Siemens' support for the international standardization of Ethernet (April 10, 1981). Ingrid Fromm, Siemens' representative to IEEE 802, quickly achieved broader support for Ethernet beyond IEEE by the establishment of a competing Task Group "Local Networks" within the European standards body ECMA TC24.<ref>{{cite web |last=Pelkey |first=James L. |date=October 11, 1988 |title=Oral History of David Liddle |url=https://archive.computerhistory.org/resources/access/text/2013/05/102746649-05-01-acc.pdf |website=computerhistory.org  |location=Mountain View, California |publisher=Computer History Museum |access-date=November 18, 2025}}</ref> In March 1984, ECMA TC24 with its corporate members reached an agreement on a standard for CSMA/CD based on the IEEE 802 draft<ref name=VonBurg2003/>{{rp|page=364}} and the International Standards organization adopted ISO 88023 for Ethernet<ref name=Breyer/>{{rp|8}} Because the DIX proposal was most technically complete and because of the speedy action taken by ECMA which decisively contributed to the conciliation of opinions within IEEE, the IEEE 802.3 CSMA/CD standard was approved in December 1982.<ref name=VonBurg2003 /> IEEE published the 802.3 standard as a draft in 1983 and as a standard in 1985.<ref>IEEE 802.3-2008, p.iv</ref>

Approval of Ethernet on the international level was achieved by a similar, cross-[[partisan (political)|partisan]] action with Fromm as the [[liaison officer]] working to integrate with [[International Electrotechnical Commission]] (IEC) Technical Committee 83 and [[International Organization for Standardization]] (ISO) Technical Committee 97 Sub Committee 6. The ISO 8802-3 standard was published in 1989.<ref>{{cite web |url=http://www.iso.org/iso/iso_catalogue/catalogue_ics/catalogue_detail_ics.htm?csnumber=16235 |title=ISO 8802-3:1989 |publisher=[[ISO]] |access-date=July 8, 2015 |archive-date=July 9, 2015 |archive-url=https://web.archive.org/web/20150709153203/http://www.iso.org/iso/iso_catalogue/catalogue_ics/catalogue_detail_ics.htm?csnumber=16235 |url-status=live }}</ref>

==Evolution==
Ethernet has evolved to include higher bandwidth, improved [[medium access control]] methods, and different physical media. The [[multidrop]] coaxial cable was replaced with physical point-to-point links connected by [[Ethernet repeater]]s or [[Network switch|switches]].<ref>{{cite web |url=http://www.networkworld.com/article/2869883/lan-wan/evolution-of-ethernet.html |publisher=[[Network World]] |author=Jim Duffy |date=April 20, 2009 |access-date=January 1, 2016 |title=Evolution of Ethernet |archive-date=June 11, 2017 |archive-url=https://web.archive.org/web/20170611140149/http://www.networkworld.com/article/2869883/lan-wan/evolution-of-ethernet.html |url-status=dead }}</ref>

Ethernet stations communicate by sending each other [[data packet]]s: blocks of data individually sent and delivered. As with other IEEE 802 LANs, adapters come programmed with globally unique 48-bit [[MAC address]] so that each Ethernet station has a unique address.{{Efn|In some cases, the factory-assigned address can be overridden, either to avoid an address change when an adapter is replaced or to use [[locally administered address]]es.}} The MAC addresses are used to specify both the destination and the source of each data packet. Ethernet establishes link-level connections, which can be defined using both the destination and source addresses. On reception of a transmission, the receiver uses the destination address to determine whether the transmission is relevant to the station or should be ignored. A network interface normally does not accept packets addressed to other Ethernet stations.{{Efn|Unless it is put into [[promiscuous mode]].|name=promiscuous}}{{Efn|Of course bridges and switches will accept other addresses for forwarding the packet.}}

An EtherType field in each frame is used by the operating system on the receiving station to select the appropriate protocol module (e.g., an [[Internet Protocol]] version such as [[IPv4]]). [[Ethernet frame]]s are said to be ''self-identifying'', because of the EtherType field. Self-identifying frames make it possible to intermix multiple protocols on the same physical network and allow a single computer to use multiple protocols together.<ref>{{cite book |author=Douglas E. Comer |author-link=Douglas E. Comer |year=2000 |title=Internetworking with TCP/IP – Principles, Protocols and Architecture |edition=4th |publisher=Prentice Hall |isbn=0-13-018380-6}} 2.4.9 – Ethernet Hardware Addresses, p. 29, explains the filtering.</ref> Despite the evolution of Ethernet technology, all generations of Ethernet (excluding early experimental versions) use the same frame formats.<ref>{{cite web|author=Iljitsch van Beijnum|title=Speed matters: how Ethernet went from 3Mbps to 100Gbps... and beyond|url=https://arstechnica.com/gadgets/2011/07/ethernet-how-does-it-work/3/|website=[[Ars Technica]]|date=July 15, 2011|access-date=July 15, 2011|quote=All aspects of Ethernet were changed: its MAC procedure, the bit encoding, the wiring... only the packet format has remained the same.|archive-date=July 9, 2012|archive-url=https://web.archive.org/web/20120709015112/http://arstechnica.com/gadgets/2011/07/ethernet-how-does-it-work/3/|url-status=live}}</ref> Mixed-speed networks can be built using Ethernet switches and repeaters supporting the desired Ethernet variants.<ref>{{citation |url=http://www.lantronix.com/resources/networking-tutorials/fast-ethernet-tutorial/ |publisher=Lantronix |access-date=January 1, 2016 |title=Fast Ethernet Turtorial |date=December 9, 2014 |archive-date=November 28, 2015 |archive-url=https://web.archive.org/web/20151128172531/http://www.lantronix.com/resources/networking-tutorials/fast-ethernet-tutorial/ |url-status=live }}</ref>

Due to the ubiquity of Ethernet, and the ever-decreasing cost of the hardware needed to support it, by 2004 most manufacturers built Ethernet interfaces directly into [[PC motherboard]]s, eliminating the need for a separate network card.<ref>{{cite web |url=http://pcquest.ciol.com/content/search/showarticle.asp?artid=63428 |title=Motherboard Chipsets Roundup |publisher=PCQuest |date=November 1, 2004 |author=Geetaj Channana |quote=While comparing motherboards in the last issue we found that all motherboards support Ethernet connection on board. |access-date=October 22, 2010 |archive-url=https://web.archive.org/web/20110708154855/http://pcquest.ciol.com/content/search/showarticle.asp?artid=63428 |archive-date=July 8, 2011 |url-status=dead }}</ref>

=== Shared medium ===
[[File:10Base5transcievers.jpg|thumb|Older Ethernet equipment. Clockwise from top-left: An Ethernet transceiver with an in-line [[10BASE2]] adapter, a similar model transceiver with a [[10BASE5]] adapter, an [[Attachment Unit Interface|AUI]] cable, a different style of transceiver with 10BASE2 [[BNC connector|BNC]] T-connector, two 10BASE5 end fittings ([[N connector]]s), an orange ''[[vampire tap]]'' installation tool (which includes a specialized drill bit at one end and a socket wrench at the other), and an early model 10BASE5 transceiver (h4000) manufactured by DEC. The short length of yellow 10BASE5 cable has one end fitted with an N connector and the other end prepared to have an N connector shell installed; the half-black, half-grey rectangular object through which the cable passes is an installed vampire tap.]]

Ethernet was originally based on the idea of computers communicating over a shared coaxial cable acting as a broadcast transmission medium. The method used was similar to those used in radio systems,{{Efn|There are fundamental differences between wireless and wired shared-medium communication, such as the fact that it is much easier to detect collisions in a wired system than a wireless system.}} with the common cable providing the communication channel likened to the ''Luminiferous aether'' in 19th-century physics, and it was from this reference that the name ''Ethernet'' was derived.<ref name="Spurgeon 2000">{{cite book |title=Ethernet: The Definitive Guide |url=https://archive.org/details/ethernetdefiniti0000spur |url-access=registration |author=Charles E. Spurgeon |publisher=O'Reilly |isbn=978-1-56592-660-8 |year=2000}}</ref>

The original Ethernet's shared coaxial cable (the shared medium) traversed a building or campus to connect every attached machine. A scheme known as [[carrier-sense multiple access with collision detection]] (CSMA/CD) governed the way the computers shared the channel. This scheme was simpler than competing Token Ring or [[Token Bus]] technologies.{{Efn|In a CSMA/CD system packets must be large enough to guarantee that the leading edge of the propagating wave of a message gets to all parts of the medium and back again before the transmitter stops transmitting, guaranteeing that [[collisions]] (two or more packets initiated within a window of time that forced them to overlap) are discovered. As a result, the minimum packet size and the physical medium's total length are closely linked.}} Computers are connected to an [[Attachment Unit Interface]] (AUI) [[transceiver]], which is in turn connected to the cable (with [[thin Ethernet]] the transceiver is usually integrated into the network adapter). While a simple passive wire is highly reliable for small networks, it is not reliable for large extended networks, where damage to the wire in a single place, or a single bad connector, can make the whole Ethernet segment unusable.{{Efn|Multipoint systems are also prone to strange failure modes when an electrical discontinuity reflects the signal in such a manner that some nodes would work properly, while others work slowly because of excessive retries or not at all. See [[standing wave]] for an explanation. These could be much more difficult to diagnose than a complete failure of the segment.}}

Through the first half of the 1980s, Ethernet's [[10BASE5]] implementation utilised a coaxial cable {{convert|0.375|in}} in diameter, later referred to as ''thick Ethernet'' or ''thicknet''. Its successor, [[10BASE2]], called ''thin Ethernet'' or ''thinnet'', used the [[RG-58]] coaxial cable. The emphasis was on making installation of the cable easier and less costly.<ref name=Hegering>{{cite book |author1=Heinz-Gerd Hegering |author2=Alfred Lapple |title=Ethernet: Building a Communications Infrastructure |publisher=Addison-Wesley |date=1993 |isbn=0-201-62405-2 |url-access=registration |url=https://archive.org/details/ethernetbuilding0000hege }}</ref>{{rp|57}}

Since all communication happens on the same wire, any information sent by one computer is received by all, even if that information is intended for just one destination.{{Efn|This ''one speaks, all listen'' property is a security weakness of shared-medium Ethernet, since a node on an Ethernet network can eavesdrop on all traffic on the wire if it so chooses.}} The network interface card interrupts the [[CPU]] only when applicable packets are received: the card ignores information not addressed to it.{{Efn|name=promiscuous}} Use of a single cable also means that the data bandwidth is shared, such that, for example, available data bandwidth to each device is halved when two stations are simultaneously active.<ref>{{citation |url=http://www.lantronix.com/resources/networking-tutorials/ethernet-tutorial-networking-basics/ |title=Ethernet Tutorial – Part I: Networking Basics |date=December 9, 2014 |publisher=Lantronix |access-date=January 1, 2016 |archive-date=February 13, 2016 |archive-url=https://web.archive.org/web/20160213014814/http://www.lantronix.com/resources/networking-tutorials/ethernet-tutorial-networking-basics/ |url-status=live }}</ref>

A collision happens when two stations attempt to transmit at the same time. They corrupt transmitted data and require stations to re-transmit. The loss of data and retransmission reduce throughput. In the worst case, where multiple active hosts connected with maximum allowed cable length attempt to transmit many short frames, excessive collisions can reduce throughput dramatically. However, a [[Xerox]] report in 1980, published in [[Communications of the ACM]], studied the performance of an existing Ethernet installation under both normal and artificially generated heavy load. The report claimed that 98% throughput on the LAN was observed.<ref>{{cite journal| author1=Shoch, John F. |author2=Hupp, Jon A. | title = Measured performance of an Ethernet local network| journal=Communications of the ACM| volume = 23| issue = 12| pages = 711–721| publisher=ACM Press| date=December 1980| issn = 0001-0782
| doi = 10.1145/359038.359044|s2cid=1002624 | doi-access = free}}</ref> This is in contrast with [[token passing]] LANs (Token Ring, Token Bus), all of which suffer throughput degradation as each new node comes into the LAN, due to token waits. This report was controversial, as modeling showed that collision-based networks theoretically became unstable under loads as low as 37% of nominal capacity. Many early researchers failed to understand these results. Performance on real networks is significantly better.<ref>{{cite journal |author1=Boggs, D.R. |author2=Mogul, J.C. |author3=Kent, C.A. |name-list-style=amp |title=Measured capacity of an Ethernet: myths and reality |date=August 1988 |publisher=ACM |url=https://dl.acm.org/doi/pdf/10.1145/52325.52347 |journal=ACM SIGCOMM Computer Communication Review| volume=18 | number=4 |pages=222–234 |doi=10.1145/52325.52347 |access-date=October 31, 2025 |archive-date=March 2, 2012 |archive-url=https://web.archive.org/web/20120302125906/http://www.hpl.hp.com/techreports/Compaq-DEC/WRL-88-4.pdf |url-status=live }}</ref>

In a modern Ethernet, the stations do not all share one channel through a shared cable or a simple [[repeater hub]]; instead, each station communicates with a switch, which in turn forwards that traffic to the destination station. In this topology, collisions are only possible if the station and switch attempt to communicate with each other at the same time, and collisions are limited to this link. Furthermore, the [[10BASE-T]] standard introduced a [[full duplex]] mode of operation, which became familiar with [[Fast Ethernet]] and the de facto standard with [[Gigabit Ethernet]]. In full duplex, a switch and a station can send and receive simultaneously, and therefore modern Ethernet networks are completely collision-free.

<gallery class="center" caption="Comparison between original Ethernet and modern Ethernet" widths="250">
File:Bustopologie.png|The original Ethernet implementation: shared medium, collision-prone. All computers trying to communicate share the same cable, and so compete with each other.
File:HUB SWITCH 6.jpg|Modern Ethernet implementation: switched connection, collision-free. Each computer communicates only with its own switch, without competition for the cable with others.
</gallery>

===Repeaters and hubs===
[[Image:Network card.jpg|thumb|A 1990s [[Industry Standard Architecture|ISA]] [[network interface card]] supporting both coaxial-cable-based [[10BASE2]] ([[BNC connector]], left) and twisted-pair-based [[10BASE-T]] ([[8P8C]] connector, right)]]
{{Main|Ethernet hub}}

For signal degradation and timing reasons, [[Coaxial cable|coaxial]] [[Ethernet segment]]s have a restricted size.<ref>{{Cite web|url=https://kb.wisc.edu/ns/page.php?id=7829|title=Ethernet Media Standards and Distances|website=kb.wisc.edu|access-date=October 10, 2017|archive-date=June 19, 2010|archive-url=https://web.archive.org/web/20100619010200/https://kb.wisc.edu/ns/page.php?id=7829|url-status=live}}</ref> Somewhat larger networks can be built by using an [[Ethernet repeater]]. Early repeaters had only two ports, allowing, at most, a doubling of network size. Once repeaters with more than two ports became available, it was possible to wire the network in a [[star topology]]. Early experiments with star topologies (called ''Fibernet'') using [[optical fiber]] were published by 1978.<ref>{{cite journal |title= Fibemet: Multimode Optical Fibers for Local Computer Networks |author1= Eric G. Rawson |author2= Robert M. Metcalfe |journal= IEEE Transactions on Communications |date= July 1978 |volume= 26 |issue= 7 |pages= 983–990 |url= http://ethernethistory.typepad.com/papers/Fibernet.pdf |doi= 10.1109/TCOM.1978.1094189 |bibcode= 1978ITCom..26..983R |access-date= June 11, 2011 |archive-date= August 15, 2011 |archive-url= https://web.archive.org/web/20110815204821/http://ethernethistory.typepad.com/papers/Fibernet.pdf |url-status= live }}</ref>

Shared cable Ethernet is always hard to install in offices because its bus topology is in conflict with the star topology cable plans designed into buildings for telephony. Modifying Ethernet to conform to twisted-pair telephone wiring already installed in commercial buildings provided another opportunity to lower costs, expand the installed base, and leverage building design, and, thus, twisted-pair Ethernet was the next logical development in the mid-1980s.

Ethernet on unshielded [[Twisted pair|twisted-pair cables]] (UTP) began with [[StarLAN]] at 1&nbsp;Mbit/s in the mid-1980s.<ref name=Breyer/>{{rp|page=12-13}} In 1987 [[SynOptics]] introduced the first twisted-pair Ethernet at 10&nbsp;Mbit/s in a star-wired cabling topology with a central hub, later called [[LattisNet]].<ref name=VonBurg2003 /><ref name="Spurgeon 2000"/>{{rp|29}}<ref>{{cite book| title = The Triumph of Ethernet: technological communities and the battle for the LAN standard| author = Urs von Burg| publisher = Stanford University Press| year = 2001| url = https://books.google.com/books?id=ooBqdIXIqbwC&pg=PA175| isbn = 0-8047-4094-1| page = 175| access-date = September 23, 2016| archive-date = January 9, 2017| archive-url = https://web.archive.org/web/20170109135141/https://books.google.com/books?id=ooBqdIXIqbwC&pg=PA175| url-status = live}}</ref> These evolved into 10BASE-T, which was designed for point-to-point links only, and all termination was built into the device. This changed repeaters from a specialist device used at the center of large networks to a device that every twisted pair-based network with more than two machines had to use. The tree structure that resulted from this made Ethernet networks easier to maintain by preventing most faults with one peer or its associated cable from affecting other devices on the network.{{citation needed|date=April 2020|reason=OK, repeaters are required to deactivate ports that send excessive collisions, such as due to internal defects, or external wiring defects. That is an important part of this statement.}}

Despite the physical star topology and the presence of separate transmit and receive channels in the twisted pair and fiber media, repeater-based Ethernet networks still use half-duplex and CSMA/CD, with only minimal activity by the repeater, primarily the generation of the [[jam signal]] in dealing with packet collisions. Every packet is sent to every other port on the repeater, so bandwidth and security problems are not addressed. The total throughput of the repeater is limited to that of a single link, and all links must operate at the same speed.<ref name="Spurgeon 2000"/>{{rp|278}}

=== Bridging and switching ===<!--[[Full-duplex Ethernet]] links here-->
[[File:Network switches.jpg|thumb|[[Patch cable]]s with [[patch field]]s of two Ethernet switches]]
{{Main|Network bridge|Network switch}}

While repeaters can isolate some aspects of [[Ethernet segment]]s, such as cable breakages, they still forward all traffic to all Ethernet devices. The entire network is one [[collision domain]], and all hosts have to be able to detect collisions anywhere on the network. This limits the number of repeaters between the farthest nodes and creates practical limits on how many machines can communicate on an Ethernet network. Segments joined by repeaters have to all operate at the same speed, making phased-in upgrades impossible.{{citation needed|date=April 2020}}

To alleviate these problems, bridging was created to communicate at the data link layer while isolating the physical layer. With bridging, only well-formed Ethernet packets are forwarded from one Ethernet segment to another; collisions and packet errors are isolated. At initial startup, Ethernet bridges work somewhat like Ethernet repeaters, passing all traffic between segments. By observing the source addresses of incoming frames, the bridge then builds an address table associating addresses to segments. Once an address is learned, the bridge forwards network traffic destined for that address only to the associated segment, improving overall performance. [[Broadcasting (networking)|Broadcast]] traffic is still forwarded to all network segments. Bridges also overcome the limits on total segments between two hosts and allow the mixing of speeds, both of which are critical to the incremental deployment of faster Ethernet variants.{{citation needed|date=April 2020}}

In 1989, [[Vanguard Managed Solutions|Motorola Codex]] introduced their 6310 EtherSpan, and [[Kalpana (company)|Kalpana]] introduced their EtherSwitch; these were examples of the first commercial Ethernet switches.{{Efn|The term ''switch'' was invented by device manufacturers and does not appear in the IEEE 802.3 standard.}} Early switches such as this used [[cut-through switching]] where only the header of the incoming packet is examined before it is either dropped or forwarded to another segment.<ref name="networkcomputing_2000">{{cite web |title=The 10 Most Important Products of the Decade |author=Robert J. Kohlhepp |date=October 2, 2000 |access-date=February 25, 2008 |publisher=Network Computing |url=http://www.networkcomputing.com/1119/1119f1products_5.html|archive-url=https://web.archive.org/web/20100105152318/http://www.networkcomputing.com/1119/1119f1products_5.html |archive-date=January 5, 2010}}</ref> This reduces the forwarding latency. One drawback of this method is that it does not readily allow a mixture of different link speeds. Another is that packets that have been corrupted are still propagated through the network. The eventual remedy for this was a return to the original [[store and forward]] approach of bridging, where the packet is read into a buffer on the switch in its entirety, its [[frame check sequence]] verified and only then the packet is forwarded.<ref name="networkcomputing_2000"/> In modern network equipment, this process is typically done using [[application-specific integrated circuit]]s allowing packets to be forwarded at [[wire speed]].{{citation needed|date=April 2020}}

When a twisted pair or fiber link segment is used and neither end is connected to a repeater, [[full-duplex]] Ethernet becomes possible over that segment. In full-duplex mode, both devices can transmit and receive to and from each other at the same time, and there is no collision domain.<ref>{{cite web |author=Nick Pidgeon |work=How Stuff Works |url=https://computer.howstuffworks.com/ethernet15.htm |title=Full-duplex Ethernet |date=April 2000 |access-date=February 3, 2020 |archive-date=June 4, 2020 |archive-url=https://web.archive.org/web/20200604085640/https://computer.howstuffworks.com/ethernet15.htm |url-status=live }}</ref> This doubles the aggregate bandwidth of the link and is sometimes advertised as double the link speed (for example, 200&nbsp;Mbit/s for Fast Ethernet).{{Efn|This is misleading, as performance will double only if traffic patterns are symmetrical.}} The elimination of the collision domain for these connections also means that all the link's bandwidth can be used by the two devices on that segment and that segment length is not limited by the constraints of collision detection.

Since packets are typically delivered only to the port they are intended for, traffic on a switched Ethernet is less public than on shared-medium Ethernet. <span id="switch_vulnerabilities">Despite this, switched Ethernet should still be regarded as an insecure network technology, because it is easy to subvert switched Ethernet systems by means such as [[ARP spoofing]] and [[MAC flooding]].</span>{{citation needed|date=April 2020}}<ref>{{Cite book|last1=Wang|first1=Shuangbao Paul|url=https://books.google.com/books?id=NFK_CyoyIGEC&pg=PT121|title=Computer Architecture and Security: Fundamentals of Designing Secure Computer Systems|last2=Ledley|first2=Robert S.|date=October 25, 2012|publisher=John Wiley & Sons|isbn=978-1-118-16883-7|language=en|access-date=October 2, 2020|archive-date=March 15, 2021|archive-url=https://web.archive.org/web/20210315204013/https://books.google.com/books?id=NFK_CyoyIGEC&pg=PT121|url-status=live}}</ref>

The bandwidth advantages, the improved isolation of devices from each other, the ability to easily mix different speeds of devices and the elimination of the chaining limits inherent in non-switched Ethernet have made switched Ethernet the dominant network technology.<ref>{{cite web |url=http://www.cisco.com/en/US/solutions/collateral/ns340/ns394/ns74/ns149/net_business_benefit09186a00800c92b9_ps6600_Products_White_Paper.html |quote=Respondents were first asked about their current and planned desktop LAN attachment standards. The results were clear—switched Fast Ethernet is the dominant choice for desktop connectivity to the network |title=Token Ring-to-Ethernet Migration |publisher=Cisco |access-date=October 22, 2010 |archive-date=July 8, 2011 |archive-url=https://web.archive.org/web/20110708160911/http://www.cisco.com/en/US/solutions/collateral/ns340/ns394/ns74/ns149/net_business_benefit09186a00800c92b9_ps6600_Products_White_Paper.html |url-status=live }}</ref>

===Advanced networking===
[[File:Coreswitch (2634205113).jpg|thumb|A core Ethernet switch]]

Simple switched Ethernet networks, while a great improvement over repeater-based Ethernet, suffer from single points of failure, attacks that trick switches or hosts into sending data to a machine even if it is not intended for it, scalability and security issues with regard to [[switching loop]]s, [[broadcast radiation]], and [[multicast]] traffic.{{citation needed|date=April 2020}}

Advanced networking features in switches use [[Shortest Path Bridging]] (SPB) or the [[Spanning Tree Protocol]] (STP) to maintain a loop-free, meshed network, allowing physical loops for redundancy (STP) or load-balancing (SPB). Shortest Path Bridging includes the use of the [[link-state routing protocol]] [[IS-IS]] to allow larger networks with shortest path routes between devices.

Advanced networking features also ensure port security, provide protection features such as MAC lockdown<ref>{{cite web |url=https://www.techrepublic.com/blog/it-security/lock-down-cisco-switch-port-security-88196/ |title=Lock down Cisco switch port security |author=David Davis |date=October 11, 2007 |access-date=April 19, 2020 |archive-date=July 31, 2020 |archive-url=https://web.archive.org/web/20200731010910/https://www.techrepublic.com/blog/it-security/lock-down-cisco-switch-port-security-88196/ |url-status=live }}</ref> and broadcast radiation filtering, use [[VLAN]]s to keep different classes of users separate while using the same physical infrastructure,<ref>{{Cite web |title=Virtual LANs (VLANS) {{!}} Department of Computer Science Computing Guide |url=https://csguide.cs.princeton.edu/access/vlans |access-date=2025-10-09 |website=csguide.cs.princeton.edu}}</ref> and use [[link aggregation]] to add bandwidth to overloaded links and to provide some redundancy.<ref>{{cite book |last1=Tholeti |first1=Bhanu Prakash Reddy |title=Handbook of Fiber Optic Data Communication |chapter=Hypervisors, Virtualization, and Networking |date=2013 |pages=387–416 |doi=10.1016/B978-0-12-401673-6.00016-7 |isbn=978-0-12-401673-6 |quote=A link aggregation, or EtherChannel, device is a network port-aggregation technology that allows several Ethernet adapters to be aggregated. The adapters can then act as a single Ethernet device. Link aggregation helps to provide more throughput over a single IP address than would be possible with a single Ethernet adapter. }}</ref>

In 2016, Ethernet replaced [[InfiniBand]] as the most popular system interconnect of [[TOP500]] supercomputers.<ref>{{cite web |url=https://www.top500.org/lists/top500/2016/06/highlights/ |title=HIGHLIGHTS – JUNE 2016 |quote=InfiniBand technology is now found on 205 systems, down from 235 systems, and is now the second most-used internal system interconnect technology. Gigabit Ethernet has risen to 218 systems up from 182 systems, in large part thanks to 176 systems now using 10G interfaces. |date=June 2016 |access-date=February 19, 2021 |archive-date=January 30, 2021 |archive-url=https://web.archive.org/web/20210130100950/https://top500.org/lists/top500/2016/06/highlights/ |url-status=live }}</ref>

==Varieties==
{{Main|Ethernet physical layer|Ethernet over twisted pair}}[[File:Ethernet Connection.jpg|thumb|An Ethernet port on a [[Laptop|laptop computer]] connected to a [[twisted pair]] cable with an [[8P8C modular connector]]]]
The Ethernet physical layer evolved over a considerable time span and encompasses coaxial, twisted pair and fiber-optic physical media interfaces, with speeds from {{nowrap|1 Mbit/s}} to {{nowrap|400 Gbit/s}}.<ref name="400Gapproval">{{Cite web |title=[STDS-802-3-400G] IEEE P802.3bs Approved! |publisher=IEEE 802.3bs Task Force |url=http://www.ieee802.org/3/400GSG/email/msg01519.html |access-date=December 14, 2017 |archive-date=June 12, 2018 |archive-url=https://web.archive.org/web/20180612144057/http://www.ieee802.org/3/400GSG/email/msg01519.html |url-status=live }}</ref> The first introduction of twisted-pair CSMA/CD was [[StarLAN]], standardized as 802.3 1BASE5.<ref>{{cite web| url = http://www.cs.nthu.edu.tw/~nfhuang/handouts/Chap04/sld022.htm| title = 1BASE5 Medium Specification (StarLAN)| date = December 28, 1996| access-date = November 11, 2014| website = cs.nthu.edu.tw| archive-date = July 10, 2015| archive-url = https://web.archive.org/web/20150710151536/http://www.cs.nthu.edu.tw/~nfhuang/handouts/Chap04/sld022.htm| url-status = live}}</ref> While 1BASE5 had little market penetration, it defined the physical apparatus (wire, plug/jack, pin-out, and wiring plan) that would be carried over to 10BASE-T through 10GBASE-T.

The most common forms used are [[Ethernet over twisted pair|10BASE-T, 100BASE-TX, and 1000BASE-T]]. All three use twisted-pair cables and [[8P8C modular connector]]s. They run at {{nowrap|10 Mbit/s}}, {{nowrap|100 Mbit/s}}, and {{nowrap|1 Gbit/s}}, respectively.<ref>IEEE 802.3 ''14. Twisted-pair medium attachment unit (MAU) and baseband medium, type 10BASE-T including type 10BASE-Te''</ref><ref>IEEE 802.3 ''25. Physical Medium Dependent (PMD) sublayer and baseband medium, type 100BASE-TX''</ref><ref>IEEE 802.3 ''40. Physical Coding Sublayer (PCS), Physical Medium Attachment (PMA) sublayer and baseband medium, type 1000BASE-T''</ref>

[[Fiber optic]] variants of Ethernet (that commonly use [[SFP module]]s) are also very popular in larger networks, offering high performance, better electrical isolation and longer distance (tens of kilometers with some versions). In general, network [[protocol stack]] software will work similarly on all varieties.<ref>IEEE 802.3 ''4.3 Interfaces to/from adjacent layers''</ref>

==Frame structure==
[[File:SMSC LAN91C110 ethernet chip.jpg|thumb|A close-up of the SMSC LAN91C110 (SMSC 91x) chip, an embedded Ethernet chip]]
{{Main|Ethernet frame}}

In IEEE 802.3, a [[datagram]] is called a ''packet'' or ''frame''. ''Packet'' is used to describe the overall transmission unit and includes the [[preamble (communication)|preamble]], [[start frame delimiter]] (SFD) and carrier extension (if present).{{Efn|The carrier extension is defined to assist collision detection on shared-media gigabit Ethernet.}} The ''frame'' begins after the start frame delimiter with a frame header featuring source and destination MAC addresses and the EtherType field giving either the protocol type for the payload protocol or the length of the payload. The middle section of the frame consists of payload data including any headers for other protocols (for example, Internet Protocol) carried in the frame. The frame ends with a 32-bit [[cyclic redundancy check]], which is used to detect corruption of [[data in transit]].<ref>{{Cite web
| url = http://standards.ieee.org/findstds/standard/802.3-2012.html
| title = 802.3-2012 – IEEE Standard for Ethernet
| date = December 28, 2012
| access-date = February 8, 2014
| publisher = IEEE Standards Association
| work = [[IEEE]]
| format = PDF
| archive-date = February 23, 2014
| archive-url = https://web.archive.org/web/20140223215930/http://standards.ieee.org/findstds/standard/802.3-2012.html
| url-status = dead
}}</ref>{{rp|sections 3.1.1 and 3.2}} Notably, Ethernet packets have no [[hop count|time-to-live field]], leading to possible problems in the presence of a switching loop.

==Autonegotiation==
{{Main|Autonegotiation}}
Autonegotiation is the procedure by which two connected devices choose common transmission parameters, e.g. speed and duplex mode. Autonegotiation was initially an optional feature, first introduced with 100BASE-TX (1995 IEEE 802.3u Fast Ethernet standard), and is backward compatible with 10BASE-T. The specification was improved in the 1998 release of IEEE 802.3. Autonegotiation is mandatory for 1000BASE-T and faster.

==Error conditions==
===Switching loop===
{{main|Switching loop}}
A switching loop or bridge loop occurs in [[computer network]]s when there is more than one [[Layer 2]] ([[OSI model]]) path between two endpoints (e.g. multiple connections between two [[network switch]]es or two ports on the same switch connected to each other). The loop creates [[broadcast radiation|broadcast storms]] as broadcasts and [[multicast]]s are forwarded by switches out every [[Computer port (hardware)|port]], the switch or switches will repeatedly rebroadcast the broadcast messages flooding the network. Since the Layer 2 header does not support a ''[[time to live]]'' (TTL) value, if a frame is sent into a looped topology, it can loop forever.<ref>{{Cite web|title=Layer 2 Switching Loops in Network Explained|url=https://www.computernetworkingnotes.com/ccna-study-guide/layer-2-switching-loops-in-network-explained.html|access-date=January 8, 2022|website=ComputerNetworkingNotes|language=en-gb|archive-date=January 8, 2022|archive-url=https://web.archive.org/web/20220108032858/https://www.computernetworkingnotes.com/ccna-study-guide/layer-2-switching-loops-in-network-explained.html|url-status=live}}</ref>

A physical topology that contains switching or bridge loops is attractive for redundancy reasons, yet a switched network must not have loops. The solution is to allow physical loops, but create a loop-free logical topology using the SPB protocol or the older STP on the network switches.<ref>{{Cite journal |last=Akhavan Sigari |first=F. |last2=Mirjalily |first2=Ghasem |last3=Saadat |first3=Reza |date=July 29, 2010 |title=Complexity reduction of the Best Multiple Spanning Tree algorithm |url=https://ieeexplore.ieee.org/document/5529599 |journal=2010 2nd International Conference on Education Technology and Computer |volume=3 |pages=V3–47–V3-51 |doi=10.1109/ICETC.2010.5529599}}</ref>

===Jabber===
A node that is sending longer than the maximum transmission window for an Ethernet packet is considered to be ''jabbering''. Depending on the physical topology, jabber detection and remedy differ somewhat.
* An [[Medium Attachment Unit|MAU]] is required to detect and stop abnormally long transmission from the [[Data terminal equipment|DTE]] (longer than 20–150&nbsp;ms) in order to prevent permanent network disruption.<ref>IEEE 802.3 ''8.2 MAU functional specifications''</ref>
* On an electrically shared medium (10BASE5, 10BASE2, 1BASE5), jabber can only be detected by each end node, stopping reception. No further remedy is possible.<ref>IEEE 802.3 ''8.2.1.5 Jabber function requirements''</ref>
* A repeater/repeater hub uses a jabber timer that ends retransmission to the other ports when it expires. The timer runs for 25,000 to 50,000 bit times for 1&nbsp;Mbit/s,<ref>IEEE 802.3 ''12.4.3.2.3 Jabber function''</ref> 40,000 to 75,000 bit times for 10 and 100&nbsp;Mbit/s,<ref>IEEE 802.3 ''9.6.5 MAU Jabber Lockup Protection''</ref><ref>IEEE 802.3 ''27.3.2.1.4 Timers''</ref> and 80,000 to 150,000 bit times for 1&nbsp;Gbit/s.<ref>IEEE 802.3 ''41.2.2.1.4 Timers''</ref> Jabbering ports are partitioned off the network until a carrier is no longer detected.<ref>IEEE 802.3 ''27.3.1.7 Receive jabber functional requirements''</ref>
* End nodes utilizing a MAC layer will usually detect an oversized Ethernet frame and cease receiving. A bridge/switch will not forward the frame.<ref>IEEE 802.1 ''Table C-1—Largest frame base values''</ref>
* A non-uniform frame size configuration in the network using [[jumbo frame]]s may be detected as jabber by end nodes.{{citation needed|date=April 2020}} Jumbo frames are not part of the official [[IEEE 802.3]] Ethernet standard.
* A packet detected as jabber by an upstream repeater and subsequently cut off has an invalid [[frame check sequence]] and is dropped.<ref>{{Citation |title=802.3-2012 - IEEE Standard for Ethernet |date=2012-12-28 |url=https://ecfsapi.fcc.gov/file/1050839507018/IEEE%20Standard%20for%20Ethernet.pdf |access-date=2015-07-05 |publisher=[[IEEE Standards Association]] |section=3.1.1 Packet format}}</ref>

===Runt frames===
* [[Ethernet frame#Runt frames|Runts]] are packets or frames smaller than the minimum allowed size. They are dropped and not propagated.<ref>{{Cite web|title=Troubleshooting Ethernet|url=http://www.cisco.com/en/US/docs/internetworking/troubleshooting/guide/tr1904.html|access-date=May 18, 2021|website=Cisco|language=en|archive-date=March 3, 2021|archive-url=https://web.archive.org/web/20210303071756/https://www.cisco.com/en/US/docs/internetworking/troubleshooting/guide/tr1904.html|url-status=live}}</ref>

==See also==
{{Div col|colwidth=22em}}
* [[5-4-3 rule]]
* [[Chaosnet]]
* [[Ethernet Alliance]]
* [[Ethernet crossover cable]]
* [[Ethernet Technology Consortium]]
* [[Fiber media converter]]
* [[ISO/IEC 11801]]
* [[Link Layer Discovery Protocol]]
* [[List of interface bit rates]]
* [[LocalTalk]]
* [[PHY]]
* [[Physical coding sublayer]]
* [[Power over Ethernet]]
* [[Point-to-Point Protocol over Ethernet]] (PPPoE)
* [[Sneakernet]]
* [[Wake-on-LAN]] (WoL)
{{Div col end}}

==Notes==
{{Notelist}}

==References==
{{Reflist}}

==Further reading==
* {{Cite web |work=Internetworking Technology Handbook |title=Ethernet Technologies |url=http://docwiki.cisco.com/wiki/Ethernet_Technologies |publisher=Cisco Systems |access-date=April 11, 2011 |archive-date=December 28, 2018 |archive-url=https://web.archive.org/web/20181228005303/http://docwiki.cisco.com/wiki/Ethernet_Technologies |url-status=dead }}
* {{cite book | author = Charles E. Spurgeon | title = Ethernet: The Definitive Guide | url = https://archive.org/details/ethernetdefiniti0000spur | url-access = registration | year = 2000 | publisher = O'Reilly Media | isbn = 978-1565-9266-08}}
* {{cite web |title= Ethernet History |work= blog |author= Yogen Dalal  |url= http://ethernethistory.typepad.com/}}

==External links==
{{Commons category|Ethernet}}
* [http://www.ieee802.org/3/ IEEE 802.3 Ethernet working group]
* [https://standards.ieee.org/ieee/802.3/6003/ IEEE 802.3-2015  – superseded]
* [https://standards.ieee.org/ieee/802.3/7071/ IEEE 802.3-2018 standard] 

{{Ethernet}}
{{Internet access}}
{{Basic computer components}}
{{Authority control}}

[[Category:Ethernet| ]]
[[Category:American inventions]]
[[Category:IEEE standards]]
[[Category:Computer-related introductions in 1980]]