Fast Ethernet refers to a set of Ethernet standards that are able to transmit data at a minimum of 100Mbps, over the original 10 Mbps Ethernet speed. Introduced in 1995, Fast Ethernet was the fastest version of Ethernet for three years. It was later superseded by Gigabit Ethernet, which could transmit data at a rate of at least 1 Gbps.

The adapter of a fast Ethernet network may be logically divided into two parts: the media access controller (MAC) which deals with the higher-level issues of availability of the medium, and the physical layer interface (PHY). The Media Independent Interface (MII), a 4-bit 25 MHz synchronous parallel interface, may be used as a means to link the MAC to the PHY.

It sets the theoretical maximum data bit rate for all versions of fast Ethernet to 100 Mbps; however, due to the necessary header and trailer for addressing and error-detection on every packet, the occasional "lost packet" due to noise, and the time spent waiting for other devices on the network to finish transmitting after each packet is sent, the data transmission rate achieved by actual networks is usually lower than the theoretical maximum.

The MII interface usually serves as a connection either within a single Network Interface Card (NIC) or between a number of NICs in a network adapter, although it may also be used as an external connection. Repeaters and hubs may also be connected to multiple PHYs for their different interfaces.

100BASE-T refers to Fast Ethernet standards for twisted pair cables. Its IEEE-designated identifier is derived from its characteristics. "100" refers to its rate of transmission of Ethernet signals at 100Mbps. "BASE" is shorthand for "baseband transmission," meaning that the medium exclusively transmits Ethernet signals. "T" denotes its use of twisted pair cables as its medium.

There are three 100BASE-T standards: 100BASE-TX, which requires two pairs of Category 5 or better cable; 100BASE-T4, which requires four pairs of Category 3 or better cable; and 100BASE-T2, which requires two pairs of Category 3 or better cable.

All three standards have a cable segment length limit of 100 meters. For their network topology, all three variations make use of a star configuration, which involves a central hub or switch to which a number of devices are connected, in essence forming a star.

There are three standards of Fast Ethernet which use fiber as their medium. These are the following: 100BASE-FX, which requires two strands of single-mode optical fiber, one for receiving (RX) and one for transmitting (TX); 100BASE-SX, which uses two-strands of multi-mode optical fiber, both for receiving and transmitting; and 100BASE-BX, which uses one strand of single-mode optical fiber and a special multiplexer which splits the signal into TX and RX wavelengths.