Ethernet
Ethernet is the dominant wired networking technology for local networks: a family of standards, published by the IEEE as 802.3, that defines how devices on a shared or switched link are identified, how data is packaged into frames, and how bits are signaled over copper cable or optical fiber. It sits at the bottom of the protocol stack, handling the hop between one device and the next, while higher layers such as IP and TCP handle the journey across many hops.
Ethernet began at Xerox PARC in 1973, when Robert Metcalfe wrote a memo describing a network in which many computers shared one coaxial cable, and with David Boggs built the first working system. The name nodded to the luminiferous "ether" once imagined to carry light. The first version ran at roughly 2.94 megabits per second, and the idea that made it work was simple: any station may transmit when the cable is quiet, and if two transmit at once, both detect the collision, stop, wait a random time, and try again. That scheme, carrier sense multiple access with collision detection (CSMA/CD), was formalized by DEC, Intel and Xerox in the 1980 "DIX" specification and then by the IEEE as 802.3 in 1983, turning a laboratory design into an open standard that many vendors could build to.
Every Ethernet transmission is a frame, a self-describing envelope with a fixed layout: a preamble for synchronization, a destination address, a source address, a type field, a payload, and a trailing checksum. The addresses are MAC addresses, 48-bit identifiers conventionally written as six pairs of hex digits, whose first three bytes identify the manufacturer and whose remainder is assigned by that manufacturer. The type field (EtherType) says which protocol rides inside, for example IPv4 or IPv6. The payload normally carries up to 1,500 bytes, a limit called the MTU, and the frame check sequence is a CRC-32 that lets the receiver discard any frame damaged in transit. Ethernet detects errors but does not repair them; recovering lost data is left to higher layers.
The shared cable that made CSMA/CD necessary has almost entirely disappeared, replaced by switches that give every device its own dedicated link. Early Ethernet used thick and thin coaxial cable, then in the late 1980s moved to twisted-pair wiring (10BASE-T) joined by hubs that simply repeated every frame to every port. Switches changed that: a switch learns which MAC address lives on which port by watching source addresses, then forwards each frame only where it needs to go. Modern full-duplex links send and receive simultaneously, so collisions no longer occur and CSMA/CD is effectively vestigial, even though the frame format it shaped survives unchanged.
Ethernet's survival is largely a story of speed increases that kept the same frame format: 10 megabits per second, Fast Ethernet at 100 megabits in the mid-1990s, Gigabit Ethernet at the end of that decade, and then 10, 25, 100 and 400 gigabits per second and beyond. Because each generation remained backward compatible in framing and auto-negotiated down to what both ends support, a single building can mix decades of equipment. Today Ethernet connects office desktops, home routers, and above all data centers, where racks of servers and AI training clusters are stitched together with very fast Ethernet or competing fabrics such as InfiniBand, and where the choice of link speed and switch design directly limits how fast GPUs can exchange gradients.
A common misconception is that a MAC address is a permanent, trustworthy identity, and that Ethernet frames reach only their intended recipient. MAC addresses are readily changed in software, and some operating systems randomize them on purpose for privacy. They also only matter within one local segment: a router rewrites the source and destination MACs at every hop, which is why a MAC address never reveals a device to a website. Likewise, on a hub or a misconfigured network, other machines can capture frames not meant for them, so Ethernet itself provides no confidentiality; encryption has to come from layers above, such as TLS, or from add-ons like MACsec.
Read Ethernet as the local delivery service of the network: a 1973 idea for letting machines politely share a wire has become a standardized frame, addressed by MAC and checked by CRC, that carries nearly every wired packet on the planet one hop at a time.