100BASE-T is any of several Fast Ethernet standards for twisted pair cables, including: 100BASE-TX (100 Mbps over two-pair Cat5 or better cable), 100BASE-T4 (100 Mbps over four-pair Cat3 or better cable, defunct), 100BASE-T2 (100 Mbps over two-pair Cat3 or better cable, also defunct). The segment length for a 100BASE-T cable is limited to 100 metres (328 ft) (as with 10BASE-T and gigabit Ethernet). All are or were standards under IEEE 802.3 (approved 1995). Almost all 100BASE-T installations are 100BASE-TX.
In the early days of Fast Ethernet, much vendor advertising centered on claims by competing standards that said vendors' standards will work better with existing cables than other standards. In practice, it was quickly discovered that few existing networks actually met the assumed standards, because 10 Megabit Ethernet was very tolerant of minor deviations from specified electrical characteristics and few installers ever bothered to make exact measurements of cable and connection quality; if Ethernet worked over a cable, no matter how well it worked, it was deemed acceptable. Thus most networks had to be rewired for 100 Megabit speed whether or not there had supposedly been CAT3 or CAT5 cable runs.