Making LAN Cables

If we apply the 568A colour code and show all eight wires, our pin-out looks like this:

Note that pins 4, 5, 7, and 8 and the blue and brown pairs are not used in either standard. Quite contrary to what you may read elsewhere, these pins and wires are not used or required to implement 100BASE-TX duplexing—they are just plain wasted.

However, the actual cables are not physically that simple. In the diagram, the orange pair of wires are not adjacent. The blue pair is upside-down. The right ends match RJ-45 jacks and the left ends do not. If, for example, we invert the left side of the 568A "straight"-thru cable to match a 568A jack—put one 180° twist in the entire cable from end-to-end—and twist together and rearrange the appropriate pairs, we get a can-of-worms.

This further emphasizes the importance of the word "twist" in making network cables which will work. You cannot use a flat-untwisted telephone cable for a network cable.