FunctionPin#Pin#RX+33RX-66

What pins are used for Fast Ethernet?

100BASE-TX (with all eight wires) is the commonly used Fast Ethernet specification today. However, it is often (lazily) referred to as simply T. Again, T is meant to refer to the category of Twisted Pair options, and TX is the specific standard that calls for using the pairs at pin-positions 1&2 and 3&6.

Does Ethernet use all 4 pairs?

Gigabit Ethernet uses all four pairs. 10 Mbps or 100 Mbps use two wire pairs. One pair sends data, the other pair receives data. Gigabit Ethernet uses all four pairs.