There are two modes, A and B, of PoE available. In mode A, pins 1 and 2 form one side of the 48 volt DC supply, and pins 3 and 6 provide the 48 V return. These are the same two pairs used for data transmission in 10Base-T and 100Base-TX, allowing the provision of both power and data over only two pairs in such networks. In mode B (most common and shown in the figure above), pins 4 and 5 form one side of the DC supply and pins 7 and 8 provide the return; these are the "spare" pairs in 10BASE-T and 100BASE-TX. Mode B, therefore, requires a 4-pair cable.