Ethernet
/ EE-ther-net /
Ethernet is the technology that almost every wired local network is built from. When you plug a computer into a wall jack with an RJ45 cable, or when your Wi-Fi access point connects back to a router, the wire underneath is speaking Ethernet. It is the common language that lets machines in the same building hand frames of data to one another.
Ethernet defines two things at once: a frame format (how the bits on the wire are laid out, with a destination and source MAC address, a type field, a payload, and a checksum) and the rules for putting those frames on the medium. The original 1970s Ethernet had every machine share one coaxial cable; they took turns using CSMA/CD (listen before you talk, and back off if two talk at once). Modern Ethernet replaces the shared cable with point-to-point links into a switch, so collisions essentially disappear and each link runs full-duplex.
Over fifty years Ethernet kept its frame format almost unchanged while its speed climbed from 10 Mbps over coax to 100 Mbps, 1 Gbps, 10 Gbps, and on to 100 Gbps and beyond over twisted pair and fiber. That stability is exactly why it won: an old program and a new program describe data the same way, even though the wire underneath is a thousand times faster. Ethernet is standardized as IEEE 802.3.
An office laptop sends a frame addressed to the printer's MAC address; the frame travels over a 1 Gbps twisted-pair link into a switch, which forwards it out only the port where the printer lives.
Same frame format, modern switched delivery — no shared cable, no collisions.
Ethernet is a layer-2 (link) technology: it moves frames between machines on the same local network using MAC addresses. It does not route across networks — that is IP's job at the layer above.