Ethernet Wiring Standards: T568A vs. T568B

Get the color order wrong by even one pin and the cable may not carry a signal at all.

What T568A and T568B Are

T568A and T568B are the two wiring orders defined by the TIA/EIA-568 standard for arranging the eight thin copper wires inside an RJ45 connector. An Ethernet cable contains four twisted pairs (eight wires), and these two standards define which colored wire goes into which pin. Both perform identically — the only difference is that the positions of the green and orange pairs are swapped.

T568A Pin Order

From pin 1: white/green, green, white/orange, blue, white/blue, orange, white/brown, brown. T568A was designed to stay compatible with older U.S. government wiring specs and legacy telephone wiring, so it still survives as the standard in some government buildings and older facilities.

T568B Pin Order

From pin 1: white/orange, orange, white/green, blue, white/blue, green, white/brown, brown. T568B traces back to an older AT&T wiring scheme and has become the de facto standard for home and office network wiring today — compared to T568A, only the positions of the 1-2 pair and the 3-6 pair are swapped.

Straight-Through Cables

A straight-through cable uses the same standard on both ends (both T568B, or both T568A) and is the most common type of Ethernet cable. It's used to connect different kinds of devices, like a computer to a router or a router to a switch. Most pre-made cables sold in stores are straight-through cables wired with T568B on both ends.

Crossover Cables and Auto-MDIX

A crossover cable wires one end as T568A and the other as T568B, and was historically used to connect two devices of the same type directly, such as computer to computer. Most modern network cards and switches now include Auto-MDIX, which automatically detects and adjusts the transmit/receive lines, so a straight-through cable works in almost every situation and dedicated crossover cables are rarely needed anymore.

What to Check When Crimping Your Own Cable

To make your own Ethernet cable, strip the outer jacket, untwist the four pairs, arrange them in your chosen order (T568A or T568B), push them fully into an RJ45 connector, and crimp with a crimping tool. Both ends must use the same standard to make a straight-through cable, and it's worth testing all eight wires with a cable tester afterward to confirm continuity.

Choosing the right cable category matters too

Beyond wiring order, the cable's category — Cat5e, Cat6, Cat6a, and so on — determines the maximum speed and bandwidth it can support. If you're running a long cable for gigabit or faster networking, checking the category rating alongside the wiring standard is worth the extra minute.

Know your networking gear before you wire anything

Understanding what device sits at each end of the cable — a switch, a router, or a hub — also affects how you plan your wiring layout. A basic grasp of what each device does helps you decide where straight-through cables are needed and where a device's uplink port matters.

Frequently Asked Questions

Which is better, T568A or T568B?

Both work identically and there's no difference in transmission performance. In practice, though, T568B is by far the more common convention, so unless you have a specific reason not to, standardizing on T568B keeps you compatible with most off-the-shelf cables and existing wiring.

What happens if I wire each end differently?

Wiring one end T568A and the other T568B makes a crossover cable. That's not ideal for typical connections between different devices, like a PC and a router, but most modern equipment has Auto-MDIX, which automatically corrects for it, so it usually still works fine.