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Straight-Through Cable Pinout

10BASE-T and 100BASE-T use two pair of wires in a UTP cable, one for each direction, as shown in Figure 2-9. The figure shows four wires, all of which sit inside a single UTP cable that connects a PC and a LAN switch. In this example, the PC on the left transmits using the top pair, and the switch on the right transmits using the bottom pair.

From

Cover of Cisco CCENT/CCNA ICND1 100-101 Official Cert Guide

Note

PC transmit on 1 and 2; Switch receives on 1 and 2. PC receives in 3 and 6; Switch transmits on 3 and 6.