Resource Cheatsheet
A+ Core 1 3.2: 568A and 568B Colours
Professor Messer A+ 220-1201 objective 3.2 — ANSI/TIA-568 and ISO/IEC 11801, the T568A and T568B pinouts, the four pins that differ, crimping checks and keystone jacks.
- Updated
- Reading time
- 6 min read
- Detail
- A+ Core 1
- Detail
- 568A and 568B Colours
On this page
In one line: ANSI/TIA-568 defines two pin-and-colour schemes for the same RJ45 connector — T568A and T568B — which differ only on pins 1, 2, 3 and 6 where orange and green swap, and the only rule that really matters is to pick one and use it at both ends of every cable.
Companion to: Professor Messer, CompTIA A+ 220-1201 Core 1, objective 3.2 (568A and 568B Colours). The full version is the 568A and 568B Colours class notes; the section overview is the Section 3 sheet.
The standards
| Body | Document | Covers |
|---|---|---|
| ISO/IEC | ISO/IEC 11801 | International cabling standard — classes of cabling |
| TIA (US) | ANSI/TIA-568 | Commercial building cabling installation — contains T568A and T568B |
Same RJ45 connector for both schemes; only the colour-to-pin assignment differs.
The pinouts
| Pin | T568A | T568B | |
|---|---|---|---|
| 1 | White/green | White/orange | differ |
| 2 | Green | Orange | differ |
| 3 | White/orange | White/green | differ |
| 4 | Blue | Blue | same |
| 5 | White/blue | White/blue | same |
| 6 | Orange | Green | differ |
| 7 | White/brown | White/brown | same |
| 8 | Brown | Brown | same |
- Only pins 1, 2, 3 and 6 differ — the orange and green pairs swap. Pins 4, 5, 7, 8 are identical.
- Memorise B only: W/Or, Or, W/Gn, Bl, W/Bl, Gn, W/Br, Br. Swap orange ↔ green for A.
- Mnemonic: A starts green, B starts orange.
- T568B is the common choice in the US and most places. Consistency across a site matters more than which one.
Reading a finished cable
| Step | Do |
|---|---|
| 1 | Hold the plug with the clip away from you, contacts facing you — pin 1 is on the left |
| 2 | Read colours left to right |
| 3 | Ignore pins 4, 5, 7, 8 — both schemes agree |
| 4 | Pin 1 white/orange → B · white/green → A |
| 5 | Check the other end matches |
Crimping
Insert the wires and look through the connector before you squeeze — nothing is lost by pulling them out and rearranging, and a crimp can’t be undone.
| Step | Detail |
|---|---|
| Strip and fan | Expose the four pairs |
| Untwist minimally | Only as far as the connector needs |
| Order and trim square | All eight reach the front together |
| Insert and inspect | Read the colour order through the plug |
| Crimp, then test | Continuity test both ends |
| Tester result | Likely cause |
|---|---|
| Two pins never light | That pair didn’t reach the front, or was trimmed short |
| Lights out of sequence | Wires in the wrong order — wrong scheme or a crossed pair |
| Works at 100, fails gigabit | Pins 4, 5, 7, 8 (blue/brown) — unused by 10/100, required by gigabit |
Don’t mix A and B
- Wire both ends the same. Never A on one end, B on the other.
- You’ll see it called a crossover online. It is the classic 10/100 crossover (crosses 1/2 with 3/6) — but it is not a gigabit crossover, which must also cross 4/5 with 7/8.
- Auto-MDI-X on modern equipment corrects crossovers in hardware anyway, so crossover cables aren’t needed.
Keystone jacks
| Point | Detail |
|---|---|
| Printed guide | A colours on one edge, B colours on the other — pick your scheme, ignore the other row |
| Layout | Often four wires each side of the jack |
| A example | Green, white/green, blue, white/blue on one side |
| B example | Orange, white/orange, blue, white/blue on one side |
| The catch | The jack’s printed order deliberately doesn’t match the pinout — it’s arranged for easy punching, and the jack routes contacts internally |
Follow the label on the jack, not the pinout table. The table is for plugs you crimp yourself.
🔐 Security notes
- Consistency makes anomalies visible: on an all-B site, a non-standard cable raises a question. Where everything is arbitrary, an added or spliced run goes unnoticed.
- Hand-made cables deserve scrutiny — a plug can be crimped onto a cable cut and rejoined mid-run. Neat, labelled terminations make inserted devices easier to spot.
- Bad terminations create intermittent faults, and teams that learn to ignore flapping alerts lose the signal that matters.
- Documentation is incident-response speed: knowing the standard, panel position and room turns “trace this cable” into a lookup.
- Unused punched-down runs are latent network access — patch only what’s in use, and keep the record current.
- Test what you install. A continuity test verifies the wiring rather than assuming the installer got it right.
Practice drills
1. Write out T568B.
White/orange, orange, white/green, blue, white/blue, green, white/brown, brown.
2. Which pins are identical between A and B?
4, 5, 7 and 8 — blue, white/blue, white/brown, brown.
3. In one sentence, how do A and B differ?
On pins 1, 2, 3 and 6, the orange and green pairs swap.
4. A cable's first two wires are white/green then green. Which scheme?
T568A.
5. Why not wire A on one end and B on the other?
Both ends should match. It crosses 1/2 with 3/6 — the old 10/100 crossover, not a gigabit crossover — and Auto-MDI-X makes crossover cables unnecessary anyway.
6. What's the single most useful habit when crimping?
Insert the wires and inspect the colour order through the connector before crimping.
7. New cable, tester shows pins 3 and 6 open. Which pair, and the fix?
The green pair in T568B. Those wires didn’t reach the front of the plug — cut the connector off and re-terminate.
8. Why doesn't a keystone jack's printed colour order match the pinout?
The layout is arranged for easy punching; the jack routes each contact internally to the correct RJ45 pin. Follow the printed guide.
Key takeaways
- ANSI/TIA-568 (US) and ISO/IEC 11801 (international) standardise cabling; both schemes use the same RJ45.
- T568B: W/Or, Or, W/Gn, Bl, W/Bl, Gn, W/Br, Br. T568A swaps orange and green.
- Only pins 1, 2, 3, 6 differ; 4, 5, 7, 8 are identical.
- Pick one scheme per site (usually B) and wire both ends of every cable the same.
- Inspect through the connector before crimping; follow the jack’s printed guide when punching down.