Ethernet /ˈiːθərnɛt/ is a family of computer networking technologies commonly used in local area networks (LANs) and metropolitan area networks (MANs). It was commercially introduced in 1980 and first standardized in 1983 as IEEE 802.3, and has since been refined to support higher bit rates and longer link distances. Over time, Ethernet has largely replaced competing wired LAN technologies such as token ring, FDDI and ARCNET.

The original 10BASE5 Ethernet uses coaxial cable as a shared medium, while the newer Ethernet variants use twisted pair and fiber optic links in conjunction with hubs or switches. Over the course of its history, Ethernet data transfer rates have been increased from the original 2.94 megabits per second (Mbit/s) to the latest 100 gigabits per second (Gbit/s), with 400 Gbit/s expected by late 2017. The Ethernet standards comprise several wiring and signaling variants of the OSI physical layer in use with Ethernet.

Systems communicating over Ethernet divide a stream of data into shorter pieces called frames. Each frame contains source and destination addresses, and error-checking data so that damaged frames can be detected and discarded; most often, higher-layer protocols trigger retransmission of lost frames. As per the OSI model, Ethernet provides services up to and including the data link layer.

Ethernet over twisted pair technologies use twisted-pair cables for the physical layer of an Ethernet computer network.

Early Ethernet cabling had generally been based on various grades of coaxial cable, but in 1984, StarLAN showed the potential of simple unshielded twisted pair by using Cat3 cable—the same simple cable used for telephone systems. This led to the development of 10BASE-T and its successors 100BASE-TX and 1000BASE-T, supporting speeds of 10, 100 and 1000 Mbit/s respectively. Often the higher-speed implementations support the lower-speed standards making it possible to mix different generations of equipment; with the inclusive capability designated 10/100 or 10/100/1000 for connections that support such combinations.

All these three standards define both full-duplex and half-duplex communication. However, half-duplex operation for gigabit speed isn't supported by any existing hardware. The higher speed 10GBASE-T running at 10 Gbit/s, consequently defines only full duplex point-to-point links which are generally connected by network switches, and doesn't support the traditional shared-medium CSMA/CD operation.