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RJ45 Pinout Standards: T568A and T568B Explained

RJ45 Pinout Standards: T568A and T568B Explained

5 November 2026

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RJ45 Pinout Standards: T568A and T568B Explained

T568A and T568B wires inside clear RJ45 plugs

An RJ45 pinout is the agreed order in which the eight wires of a twisted-pair cable are placed in the connector. Two standards define that order, T568A and T568B. They differ only in the positions of the green and orange pairs, and in UK commercial installations T568B is the usual choice. What matters most is that you pick one and use it everywhere on the site.

This guide gives you both colour codes pin by pin, explains how to read pin numbers correctly, shows what goes wrong when ends are mixed, and covers how to check a finished termination. It is written for office managers, business owners and IT leads who want to understand what their cabling contractor is doing, or who need to specify the work properly.

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TL;DR

  • RJ45 is the connector; Ethernet is the network it carries. The pinout decides which wire sits in which of the eight positions.
  • T568A and T568B are identical except that the orange and green pairs swap places. Neither is faster than the other.
  • T568B is the de facto choice for commercial installations in the UK. Match whatever your building already uses.
  • Use one standard for sockets, patch panels and plugs, and wire both ends of a patch cable the same way.
  • Test every run with a proper wire-map test, because a cable can pass a basic continuity check and still be faulty.

Table of Contents

RJ45 and Ethernet: what is the difference?

People often use the two words as if they were one thing. They are not. RJ45 is the connector system used at the ends of Ethernet cables, while Ethernet is the wired connection those cables are used for. The plug on the end of the lead from your router is an RJ45 plug. The technology that carries your data over that lead is Ethernet.

Technically, an RJ45 connector is a mechanically keyed variant of an 8P8C connector, meaning eight positions and eight contacts, and it terminates eight colour-coded wires. In everyday trade use, “RJ45” has become the name for any such plug or socket, and that is how this article uses it.

The eight wires are arranged as four twisted pairs. Twisting each pair is what allows the cable to carry data cleanly over distance, so the pinout is not just about colour. It is about keeping each pair together on the right pair of pins.

How to read RJ45 pin numbers

Before any colour code makes sense, you need to know which pin is which. Hold the plug with the retaining clip facing away from you and the gold contacts facing up. Pin 1 is on the left and pin 8 is on the right.

That is the view of a plug, looking at the contacts from the front. On a wall socket or keystone jack, you do not count pins at all. Instead, follow the colour code printed on the module itself, because the physical layout of the insulation-displacement terminals does not mirror the order of a loose plug. The manufacturer of the module prints a code for you to follow rather than the order you would use on a plug.

Two habits save a lot of rework:

  1. Always pick one reference view and stick with it. Most mistakes happen when someone counts from the wrong side.
  2. Read the code on the jack, not from memory. Printed labels often show both A and B, so check which one your site is using before you punch down.

The T568A and T568B pinout chart

The connection of RJ45 terminations uses T568A and T568B, defined in the US standard ANSI/TIA/EIA-568.2-D, which is the reference UK installers work from for this detail. The colour sequences are as follows, as published by a UK cable accessories manufacturer:

Pin T568A T568B
1 White/Green White/Orange
2 Green Orange
3 White/Orange White/Green
4 Blue Blue
5 White/Blue White/Blue
6 Orange Green
7 White/Brown White/Brown
8 Brown Brown

Look at the table and the pattern is plain. Pins 4, 5, 7 and 8 are the same in both. Only pins 1, 2, 3 and 6 change, and they change by trading the green pair and the orange pair. In T568B, pins 1 and 2 carry TX+ and TX−, while pins 3 and 6 carry RX+ and RX−, so the pins that carry the transmit and receive signals are the ones the standards rearrange.

A simple way to remember the difference: in T568A the green pair comes first, and in T568B the orange pair comes first. Pin 1 tells you which one you are looking at.

T568A vs T568B: how they differ

The key point is that the only difference is that the orange and green pairs are interchanged, and all other wires connect to the same pins. There is no hidden trade-off behind that swap.

Does performance change?

No. Both standards give the same transmission performance. The choice comes down to the application, backward compatibility, consistency and manufacturer preference, according to Fluke Networks. Other UK guidance agrees that neither wiring scheme offers superior performance or speed, so the decision is operational: match existing infrastructure or the site specification.

That is worth repeating to anyone who tells you one is “better” for a faster office network. A gigabit link on T568A and a gigabit link on T568B behave the same, provided each is wired consistently and the cable itself is up to the job.

Why consistency is what counts

The real risk is not choosing the “wrong” standard. It is ending up with a building where sockets are wired one way, patch panels another, and a drawer of hand-made leads a third. Fault-finding in that building takes far longer than it should, because nobody can trust the colours they see.

Which standard should you use in the UK?

T568B is the de facto choice for commercial installations in the UK, while T568A is specified in some government and residential standards. For a typical office, shop, clinic or warehouse in Essex or London, T568B is therefore the sensible default.

The rules of thumb a good installer follows:

  • Existing building: match what is already there. If the previous installer used T568A throughout, carry on with T568A.
  • New installation: specify T568B in the brief unless a client or public-sector specification names T568A.
  • Whole site: use one standard throughout, including sockets, patch panels and plugs.
  • Document it: write the standard on the patch panel schedule or in the handover pack, so the next engineer does not have to guess.

If you are moving into new premises, ask which standard the existing cabling follows before you order extra drops. It is a one-line question that avoids a mixed system.

What happens if the two ends do not match?

A standard patch lead, also called a straight-through cable, uses identical pin assignments at both ends. Pin 1 goes to pin 1, pin 2 to pin 2, and so on. That is the polarity rule in simple form: the wire at pin 1 on one end must arrive at pin 1 on the other.

When one end is T568A and the other T568B, the cable is no longer straight through. For a straight-through link, the intended pin assignment should be maintained at both ends, and T568A at one end with T568B at the other crosses the orange and green pairs. In practice, mixing the two can compromise signals and make troubleshooting more difficult.

There is one deliberate use of this. A crossover cable is made by using T568A at one end and T568B at the other. Crossover leads were once needed to link two like devices directly, but they are rarely wanted in a modern office. An accidental one, though, is a common cause of a dead or flaky socket: the plug looks fine, the link light may or may not appear, and nobody can see why.

If you find a cable behaving oddly, look at the plugs at both ends side by side. If pin 1 is white/orange on one and white/green on the other, you have found the fault.

What happens if the two ends do not match?

Common termination faults

The colour order is only one of the ways a termination can go wrong. The ones that cause the most service calls are:

  • Crossed pairs: one end wired as T568A and the other as T568B, as above.
  • Excessive untwist: stripping back too much of each pair at the plug or jack. Along with poor termination or cable damage, it can cause a slow or unstable connection.
  • Split pairs: the right colours reach the right pins but the wrong wires are paired together. These are the hardest to spot, and the next section explains why.
  • Opens and shorts: a wire not making contact, or two wires touching.
  • Reversed pairs: the two wires of one pair swapped at one end.

Most of these are invisible to the eye once a plug is crimped. That is why a finished run needs a test result, not just a visual check.

How to check a termination is correct

A wire-map test confirms that each wire reaches the matching pin at the far end. Wire-map testing checks continuity to the corresponding pin and detects opens, shorts, reversed pairs, crossed pairs and split pairs.

The limit of the cheapest testers is worth knowing. A simple DC continuity test can pass a split pair, which a near-end crosstalk test will fail significantly. In other words, a cable that lights up every LED on a basic tester can still be the reason a link runs slowly or drops under load.

The same principle applies in other fields where multi-wire assemblies are checked before they go into service. Bench wiring harness testing works on the idea that a pass or fail only means something if the test measures what actually goes wrong, and cable testing is no different.

A sensible checking routine for a new or repaired run:

  1. Confirm the standard in use on the site before you terminate anything.
  2. Terminate to the code printed on the jack or panel, not from memory.
  3. Keep the untwisted length of each pair as short as the connector allows.
  4. Run a wire-map test on every drop and keep the result.
  5. Label both ends of the cable and record the port number against the socket.

For a handful of leads, a basic tester is a reasonable first check. For a floor of data points or a new office fit-out, you want a recorded result for each run, because that record is how you prove the installation is sound later.

Cabling in a building: the wider requirements

Pinout is one small part of installing data cabling in a workplace. Cables run through walls, ceilings and containment, and the electrical rules around them matter. BS 7671 requirements apply to the installation of network data cables within a building, which is a good reason to have the work planned and carried out by a competent installer rather than improvised.

If you are planning new data points, wifi access points or a move, the pinout standard should be settled at design stage, alongside cable category, routes, patch panel layout and labelling. Our network cabling service covers professional installation, and our wifi survey page explains how coverage is planned so the cable runs behind your access points end up in the right places.

Our perspective on pinouts

Most of the cabling problems we are called to are not exotic. A new socket was added by someone using a different colour code from the rest of the building, or a lead was made up on the day and never tested. The fix is nearly always the same: agree the standard, write it down, and test every run.

For business premises we would default to T568B unless the site already uses T568A. The point is never to be clever. The point is that the next engineer, whether that is us or anyone else, opens the patch panel and finds exactly what the paperwork says.

We also think cabling deserves the same care as the services running over it. Fast broadband, hosted telephony or a well-designed wifi network all depend on the copper in the walls being terminated properly. We install with minimal disruption to your working day and with UK-based support if something needs attention afterwards.

Paul

Need cabling installed or checked?

If you manage premises in Essex, London or the South East and want data points added, a messy comms cabinet put right, or a new office wired from scratch, we can help. We install professional network cabling, design wifi to suit your layout, and supply business broadband, leased lines, VoIP and mobile services without long-term contract lock-in.

Start with our network cabling installation page to see what a project involves.

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If the cabling is part of a bigger upgrade, our business broadband plans show what is available for your premises.

FAQ

What is the pinout of an Ethernet cable?

An Ethernet cable with RJ45 plugs has eight wires in eight pin positions, wired to either T568A or T568B. Counting from the left with the clip facing away and contacts up, pin 1 is on the left and pin 8 on the right. In T568B, pin 1 is white/orange and pin 2 is orange. In T568A, pin 1 is white/green and pin 2 is green.

Is an RJ45 the same as Ethernet?

No. RJ45 is the connector fitted at the ends of the cable, and Ethernet is the wired networking technology that cable carries. You can have an RJ45 connector used for other purposes, and Ethernet can run over other media, but in an office the two almost always appear together.

Does it matter whether I use T568A or T568B?

For performance, no. The two give the same transmission performance and differ only in swapping the orange and green pairs. What matters is consistency. Use one standard across sockets, patch panels and plugs, and match the existing wiring in your building. In UK commercial work, T568B is the usual choice.

What happens if one end is T568A and the other T568B?

The cable becomes a crossover, with the orange and green pairs crossed between the two ends. A normal patch lead should be wired identically at both ends. A mixed lead can give a poor or dead connection and makes faults harder to trace, so remake it to a single standard.

How do I test a termination?

Use a wire-map tester that checks every wire reaches the matching pin at the far end. It should detect opens, shorts, reversed, crossed and split pairs. For new installations, record a result for each run so you have proof the work was done properly.

Sources