What are T568A and T568B, and why must both ends of a straight-through cable use the same one?

T568A and T568B are two TIA/EIA wiring standards that define the order the eight wires sit in an RJ-45 plug; they swap the green and orange pairs. A straight-through cable uses the same standard on both ends so each pin maps straight across.

T568A and T568B are pinout conventions for terminating UTP into an RJ-45 connector. They are electrically equivalent; the only difference is that the green and orange pairs are swapped between them.

• Straight-through: same standard on both ends (A-A or B-B). Pin 1 connects to pin 1, etc. — used to connect unlike devices (host to switch/router).
• Crossover: T568A on one end, T568B on the other. The swap re-routes the transmit and receive pairs so two like devices (host-to-host, switch-to-switch) can talk.

Gotcha: mixing A on one end and B on the other by accident turns a cable you wanted as straight-through into a crossover — a classic cabling mistake. Modern NICs with Auto-MDIX hide this by auto-detecting, which is why crossover cables are now considered legacy.

Go deeper:

• ANSI/TIA-568 — Wikipedia — the standard that defines the T568A and T568B pin-and-pair assignments.

• Ethernet over twisted pair — Wikipedia — how an A-on-one-end/B-on-the-other mix yields a crossover, and the role of Auto-MDIX.

• Straight-through, crossover & rollover pinouts — Computer Cable Store — the actual pin maps for each wiring scheme.