Editing Ethernet (section)

=== 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>