RJ45 Wiring Diagram: T568A vs T568B and What Goes Wrong If You Mix Them

The fault almost never shows up on the bench. A patch cord or keystone jack terminated from memory passes a quick continuity beep, gets installed, and works fine at 100 Mbps — then the link quietly drops to a slower negotiated speed, fails a certification scan, or starts dropping PoE-powered devices once real traffic hits it. The usual culprit is a wiring mistake the installer never noticed: one end punched down to T568A and the other to T568B, or worse, a split pair that looks correct but ruins crosstalk performance.

A correct RJ45 wiring diagram is the cheapest insurance against all of it. This guide gives you the T568A and T568B pinout side by side, the full RJ45 color code, the pin-to-pair mapping that actually matters for Gigabit and PoE, a step-by-step termination walk-through for both crimp plugs and keystone jacks, and the handful of mistakes that cause most field failures — plus how to catch them before the cable goes in the wall.

What an RJ45 connector actually is

The connector everyone calls "RJ45" is technically an 8P8C modular connector — eight positions, eight contacts — used to terminate four-pair twisted-pair Ethernet cable such as Cat5e, Cat6 and Cat6a. The pin order isn't arbitrary: it's defined by the ANSI/TIA-568 cabling standard, which specifies two interchangeable pin-to-pair assignments, T568A and T568B. Both terminate all eight conductors and both perform identically when used consistently — the only thing that separates them is which colored pair lands on which pins.

That "used consistently" part is where installations go wrong, so the pinout itself is where we start.

RJ45 pinout: T568A vs T568B color code

An 8P8C connector holds four twisted pairs. With the locking tab facing away from you and the gold contacts up, pin 1 is on the left. The table below is the complete RJ45 wiring chart — the difference between the two standards is that the orange and green pairs swap places; the blue and brown pairs stay put in both.

Pin/pair mapping per ANSI/TIA-568.2-D. Pins 4-5 (blue) and 7-8 (brown) are identical in both standards; only pins 1-2 and 3-6 (orange/green) are reversed.

Why the pairing matters more than the color: each signal travels on a balanced pair, and the cable's noise rejection depends on keeping both conductors of a pair together on adjacent or paired pins. In T568B, the orange pair sits on pins 1-2 and the green pair on 3-6; T568A is the mirror image. Gigabit Ethernet (1000BASE-T) and modern PoE (IEEE 802.3bt) use all four pairs — the old shortcut that "only pins 1, 2, 3 and 6 matter" was true for 10/100BASE-T and is wrong for anything running today.

Which standard should you wire to?

Functionally, neither is faster. The decision is about consistency across a site, not performance:

• T568B is the most common choice in U.S. commercial and enterprise installs. If you're matching an existing building, this is usually what you'll find.
• T568A is specified for some U.S. residential and federal/government work (it maintains backward compatibility with older USOC voice wiring), and is more common in installations following TIA-570 residential guidance.

How to wire an RJ45 connector (plug & keystone jack)

The procedure differs slightly between a crimp-on plug (for patch cords) and a punch-down keystone jack (for wall plates and patch panels), but the prep is the same.

Crimping an RJ45 plug

1. Strip about 1 inch (25 mm) of the outer jacket without nicking the conductors.
2. Untwist the four pairs and arrange the eight wires in your chosen standard's color order (T568A or T568B from the chart above).
3. Flatten and straighten the wires, then trim them square to roughly 0.5 inch (13 mm) — short enough that the jacket seats inside the connector and the conductors reach the gold contacts.
4. Slide all eight wires into the plug, contacts up, confirming the color order at the tip before you crimp.
5. Crimp with an 8P8C ratchet tool until the contacts pierce the conductors and the strain relief grips the jacket.

Punching down a keystone jack

1. Strip 1-2 inches of jacket and fan the pairs toward the jack's color-coded IDC slots.
2. Match each conductor to the A or B color legend printed on the jack — most keystone jacks label both standards, so use only the row that matches your site standard.
3. Seat each wire and punch it down with a 110 tool, which both terminates and trims the excess in one motion.
4. Keep the untwist length as short as possible — see the warning below.

Straight-through vs crossover wiring

A straight-through cable uses the same standard on both ends (B-to-B or A-to-A) and is what you want for nearly everything: PC to switch, switch to wall jack, access point to PoE injector. A crossover cable deliberately uses T568A on one end and T568B on the other, swapping transmit and receive pairs to connect two like devices directly (older switch-to-switch or PC-to-PC links).

Here's the practical takeaway: almost all current switches, routers and NICs support Auto-MDI-X, which detects and corrects the connection automatically — so dedicated crossover cables are largely obsolete. That means an accidental A/B mismatch usually still links on modern gear, which is exactly why the mistake goes unnoticed until you connect older or fixed-MDI equipment that won't auto-correct.

Common mistakes & how to verify your termination

• Split pairs. The classic silent killer: the eight wires are in the right pin positions by color, but conductors from two different twisted pairs are used for one signal pair. A basic continuity tester shows all eight pins connected and passes — but crosstalk performance collapses and Gigabit links degrade. Only a wire-map or certification tester that checks pairing catches this.
• Mixed standards end-to-end. T568A on one end, T568B on the other, when you wanted straight-through. Modern Auto-MDI-X hides it; legacy gear won't.
• Too much untwist. Unwinding more than ~13 mm of pair at the termination degrades NEXT, especially on Cat6/Cat6a.
• Wrong IDC for the conductor. Using a jack or plug rated for stranded conductors on solid in-wall cable (or vice versa) leads to intermittent contact.

To verify: run a wire-map test first to confirm pin-to-pin continuity, correct pairing, and the absence of opens, shorts and reversals. For permanent links and any Cat6/Cat6a run, follow with a certification test that measures NEXT, return loss and insertion loss against the category limits — that's the only way to confirm a termination meets the standard rather than merely "lights up."

What you need to terminate RJ45

Highlighted row indicates a category Cable Leader stocks for this job.

Frequently asked questions

What is an RJ45 connector?

An RJ45 connector is an 8-position, 8-contact (8P8C) modular plug or jack used to terminate four-pair twisted-pair Ethernet cable such as Cat5e, Cat6 and Cat6a. Its pin order is defined by the ANSI/TIA-568 standard, which specifies two interchangeable color schemes, T568A and T568B. It's the standard connector for wired Ethernet on computers, switches, routers and wall jacks.

What is the difference between T568A and T568B?

T568A and T568B are two valid pin-to-pair color assignments for an RJ45 connector. The only difference is that the orange and green pairs are swapped: the blue pair (pins 4-5) and brown pair (pins 7-8) are identical in both. Electrically they perform the same, so the choice is purely about wiring every connector on a site to one consistent standard.

How do you wire an RJ45 connector?

Strip about an inch of jacket, untwist the four pairs, and arrange the eight wires in T568A or T568B color order. Trim them square to roughly half an inch, insert them into the plug with the contacts facing up, confirm the color order at the tip, and crimp with an 8P8C tool. For keystone jacks, fan the pairs into the matching color-coded slots and punch each down with a 110 tool.

Can I use T568A on one end and T568B on the other?

Only if you specifically want a crossover cable, which swaps the transmit and receive pairs to link two like devices directly. For a normal straight-through cable — the kind used for almost all modern connections — both ends must use the same standard. An accidental A/B mismatch often still works on gear with Auto-MDI-X, which is exactly why this mistake goes unnoticed.

Does RJ45 wiring order matter for Gigabit and PoE?

Yes. Gigabit Ethernet (1000BASE-T) and modern Power over Ethernet (IEEE 802.3bt) use all four pairs, so the old idea that only pins 1, 2, 3 and 6 matter no longer applies. All eight conductors must be correctly paired and terminated, and excessive untwist or split pairs will degrade both data throughput and PoE delivery.

How do I know if my cable is wired correctly?

Run a wire-map test to confirm every pin connects end to end, the pairs are correct, and there are no opens, shorts or reversals — this catches split pairs that a simple continuity beeper misses. For permanent Cat6/Cat6a links, follow with a certification test that measures crosstalk and loss against the category limits, which is the only way to confirm the run meets the standard.

References

1. ANSI/TIA-568.2-D, Balanced Twisted-Pair Telecommunications Cabling and Components Standard — Telecommunications Industry Association. tiaonline.org. Accessed: June 2026.
2. IEEE 802.3, Standard for Ethernet — IEEE Standards Association. standards.ieee.org. Accessed: June 2026.
3. IEEE 802.3bt, Power over Ethernet — IEEE Standards Association. standards.ieee.org. Accessed: June 2026.
4. Telecommunications Distribution Methods Manual (TDMM) — BICSI. bicsi.org. Accessed: June 2026.
5. Twisted-pair cabling certification — wire map, NEXT and insertion loss testing — Fluke Networks. flukenetworks.com. Accessed: June 2026.