RJ45 colour code and pinout (T568A and T568B)

An RJ45 connector, properly called 8P8C (eight positions, eight contacts), has eight contacts numbered 1 to 8. Hold a jack with the latch slot at the bottom and look into it: pin 1 is on the left. Hold a plug with the gold contacts facing you and the clip away from you: pin 1 is again on the left.

Twisted-pair Ethernet uses the eight wires as four twisted pairs. 10BASE-T and 100BASE-TX use only pins 1, 2, 3 and 6; gigabit and faster use all four pairs in both directions at once. Power over Ethernet adds DC power on the same pairs. Tap a pin in the diagram above to see its signal and colour in each scheme.

RJ45 colour code: T568A vs T568B

The RJ45 color code comes from the TIA-568 cabling standard, which defines two wiring patterns, T568A and T568B. Both put the four twisted pairs on the same pins and differ only in which pair goes where: the orange and green pairs swap places on pins 1, 2, 3 and 6, while blue stays on 4 and 5 and brown on 7 and 8. Electrically they perform the same.

T568B colour order, pin 1 to 8

1. white-orange
2. orange
3. white-green
4. blue
5. white-blue
6. green
7. white-brown
8. brown

T568A colour order, pin 1 to 8

1. white-green
2. green
3. white-orange
4. blue
5. white-blue
6. orange
7. white-brown
8. brown

A memory aid: in both schemes the striped wire of each pair comes first, and the blue pair sits in the middle with its solid wire first (pin 4 blue, pin 5 white-blue). The pair on pins 3 and 6 straddles the blue pair, which is the detail most people get wrong.

Which should you use, T568A or T568B?

Either works, as long as both ends of every cable and every jack in a link use the same scheme. If you are adding to an existing installation, match what is there: look at the colour on pin 1 of an existing jack or plug. White-orange means T568B; white-green means T568A.

• T568A is the pattern TIA-568 names as preferred, and US federal government contracts require it. It is also backward-compatible with older two-pair telephone wiring, which is why residential cabling guides favour it.
• T568B is the long-established habit in North American commercial installs, and many factory patch cords are wired to it.

A cable with T568A at one end and T568B at the other is a crossover cable, not a mistake that matters much today: modern ports with auto-MDI-X correct it automatically. In a structured cabling system, though, mixed schemes make troubleshooting harder, so pick one and label it.

How to crimp an RJ45 connector

You need a cable stripper or sharp blade, flush cutters, a crimp tool that fits your plugs, the plugs themselves and a cable tester.

1. Cut the cable square and slide on the strain-relief boot first, if you use one.
2. Strip about 25 mm (1 in) of jacket without nicking the conductors. Remove the rip cord and, on Cat6, the plastic spline.
3. Untwist the pairs and lay them out in T568B (or T568A) order, pin 1 on the left with the plug’s clip facing away from you.
4. Flatten and trim the wires straight across so about 13 mm (½ in) sticks out of the jacket, enough to reach the end of the plug with the jacket inside it.
5. Push the wires in until every conductor touches the end of the plug; look through the tip to check the colour order.
6. Check that the jacket sits under the strain-relief wedge, then crimp firmly until the tool releases.
7. Test it. A wire-map tester confirms each pin, and shows opens, shorts, reversed and crossed pairs.

Pass-through plugs, where the wires come out of the front and are cut off after crimping, make it easier to get the order right. Plugs are made for solid or stranded conductors and for a range of conductor and jacket sizes; Cat6 cable, with thicker 23 AWG conductors, needs plugs rated for it. A plug that does not match the cable is the commonest cause of a connection that works on the bench and fails later.

Straight-through vs crossover cables

On 10BASE-T and 100BASE-TX, a computer’s port transmits on pins 1 and 2 and receives on 3 and 6, while a switch port does the opposite, so a straight cable connects transmit to receive. Two computers wired directly once needed a crossover. Gigabit and faster use all four pairs in both directions at once and handle the swap in the port electronics.

Cat5e, Cat6, Cat6A and Cat8 with RJ45

The colour code and pin order are the same for every category of twisted-pair cable. What changes is how fast the cable can carry data and how carefully it has to be terminated:

The 100 m channel is 90 m of fixed cable between the patch panel and the wall jack plus up to 10 m of patch cords in total. Keep each pair twisted to within 13 mm (½ in) of the termination, avoid bending the cable tighter than four times its diameter, and keep it away from fluorescent fittings and power cables where you can. A mains cable and a data cable in the same containment usually need a barrier or separation; follow the local wiring rules.

RJ45 jacks, keystones and patch panels

Wall jacks and patch panels are terminated with a punch-down tool rather than crimped. Each keystone module has colour labels for both schemes, usually marked A and B; follow one set. Punch each conductor into its slot with the tool’s cutting edge facing the waste end, keep the twists right up to the slot and keep the jacket close to the module. The jack’s pin 1 is on the left when you look into it with the latch slot at the bottom, as in the diagram at the top of this page.

Testing an Ethernet cable

• Wire-map tester. A two-part tester lights each pin in turn at both ends. It finds opens, shorts, crossed wires and reversed pairs.
• Split pairs. If pin 3 and pin 6 are not the two wires of one pair, every pin still connects end to end, so a basic tester passes the cable, but crosstalk ruins it at speed. Only a qualification or certification tester sees a split pair; checking the colour order is the cheap defence.
• A link stuck at 100 Mb/s usually means one of the blue or brown conductors is open or badly crimped: 100BASE-TX needs only the orange and green pairs, gigabit needs all four.

Power over Ethernet pins

PoE sends DC power over the same cable. Mode A puts it on the data pairs (pins 1, 2, 3 and 6), Mode B on pins 4, 5, 7 and 8, and the higher-power 802.3bt types use all four pairs. A powered device must accept either mode and either polarity.

The gap between the two power columns is the allowance for loss in 100 m of cable. Bundles of PoE cables run warm, which is one reason to choose 23 AWG Cat6 or Cat6A for high-power devices. For the cable’s own resistance and the volts it loses, the same Ohm’s law arithmetic applies as for any other circuit.

RJ45 vs RJ11

An RJ11 telephone plug is narrower, with six positions and usually two or four contacts. It will sit in an RJ45 jack, touching the centre pins 4 and 5, but an RJ45 plug will not go into an RJ11 socket. Strictly, the Ethernet connector is an 8P8C modular connector; “RJ45” was originally a telephone wiring arrangement, and the name stuck to the connector.

Key facts

• The connector is properly an 8-position, 8-contact (8P8C) modular plug and jack; 'RJ45' is the everyday name. Contacts are numbered 1 to 8.
• Finding pin 1 on a plug: hold it with the clip pointing away from you and the contacts facing you, tip up - pin 1 is on the left. On a jack viewed from the front with the latch slot at the bottom, pin 1 is also on the left.
• T568A and T568B differ only by swapping the orange and green pairs on pins 1, 2, 3 and 6. Blue stays on 4-5 and brown on 7-8 in both. Either works electrically; what matters is using the same scheme at both ends.
• TIA-568 names T568A as the preferred pattern and US federal contracts require it; T568B is equally valid and also widely used.
• 10BASE-T and 100BASE-TX use only two pairs: pins 1-2 and 3-6. A PC/router port (MDI) transmits on 1-2 and receives on 3-6; a switch port (MDI-X) does the opposite.
• 1000BASE-T (and 2.5G/5G/10GBASE-T) uses all four pairs at once, in both directions: pair A = pins 1-2, B = 3-6, C = 4-5, D = 7-8.
• Pins 3 and 6 are one twisted pair that straddles the centre pair on 4-5. If the conductors of two pairs get mixed (a 'split pair'), every pin still connects end to end, so a simple continuity check passes, but near-end crosstalk fails badly.
• Straight-through cable: same scheme both ends, pin n to pin n. Crossover for 10/100: T568A at one end and T568B at the other (1↔3, 2↔6). A full gigabit crossover also swaps 4↔7 and 5↔8.
• Most modern ports have auto-MDI-X and swap pairs automatically, so straight cables work for every connection; at gigabit speed an auto-MDI-X PHY swaps pairs C and D as well as A and B.
• PoE Alternative A (PD side: Mode A) sends power over the data pairs 1-2 and 3-6; Alternative B (Mode B) uses 4-5 and 7-8. A powered device must accept either pairset and either polarity.
• PoE power by IEEE type: Type 1 / 802.3af = 15.4 W at the PSE (about 13 W at the device); Type 2 / 802.3at 'PoE+' = 30 W (25.5 W at device); Type 3 / 802.3bt = 60 W (51 W); Type 4 / 802.3bt = 90 W (71.3 W at device).
• Type 1 and 2 power over two pairs only. 802.3bt (Types 3 and 4) can energise all four pairs, and must do so for Class 5 and above (45 W+).
• PSE output voltage: 44-57 V for Type 1, 50-57 V for Types 2 and 3, 52-57 V for Type 4. 57 V is the ceiling for every type.
• PoE budgets assume a worst-case loop resistance of 12.5 Ω per pairset (6.25 Ω when both pairsets share the load); the gap between PSE and device wattage is that allowance for cable loss.
• Figures such as '100 W PoE' come from proprietary or marketing schemes, not IEEE 802.3; the standard tops out at 90 W sourced (Class 8).

Sources

Pin assignments were checked against these references on 2026-10-14. The diagram is our own drawing.

• Panduit installation instructions PN375C - TX6 PLUS shielded modular plug, T568B & T568A wiring schemes
• Fluke Networks knowledge base - Differences between wiring codes T568A vs T568B
• Fluke Networks blog - Wire Map Testing: It's Not All About Color
• Fluke knowledge article KA-04981 (T568A vs T568B)
• Cisco Catalyst 3750-X and 3560-X Hardware Installation Guide, Appendix B - Connector and Cable Specifications
• Ethernet Alliance white paper - Overview of 802.3bt Power over Ethernet standard, v2.1 (Dec 2019)
• Linux kernel documentation - PSE Power Interface (PSE PI)
• Texas Instruments technical article SSZT699 - The New PoE Standard: It's Almost Here
• Texas Instruments DP83867 Gigabit Ethernet PHY datasheet