Wire a Cat6 cable

An Ethernet cable is eight thin copper wires twisted into four pairs inside a plastic jacket, with a plug on each end. Everyone calls the plug RJ45. Strictly it's an 8P8C modular connector (eight positions, eight contacts), and you'll see both names on packets of plugs.

Cat6 is the cable's category, a rating for how fast a signal it can carry cleanly. Cat6 carries gigabit Ethernet up to 100 m, and 10 Gb/s up to about 55 m. Cat5e also does gigabit, and Cat6A does 10 Gb/s over the full 100 m. The way you terminate them is the same.

There are two different jobs, and they want different cable:

Pass-through or standard plugs?

Pass-through plugs have holes in the front. The wires go all the way through and stick out, so you can check the order before you crimp, and the crimper's blade trims them flush. Standard plugs have a closed front: you trim the wires to length first, then push them in until they touch the end. Pass-through plugs are much easier to get right the first time. Some Cat6 plugs come with a small load bar insert instead: you thread the wires through it in order, slide it down, trim, and push the bar into the plug.

Use T568B at both ends. It's the most common order for patch cables, and Step 10 explains the other one, T568A. This picture is the whole step:

A way to remember it

Orange first, brown last, and every pair goes stripe, then solid, except the blue pair in the middle (pins 4 and 5), which goes solid, then stripe. The green pair is split around it, on pins 3 and 6. Say it as pairs: orange · green-stripe · blue, blue-stripe · green · brown.

The split green pair is the one thing beginners “tidy up”, and it's the one mistake a basic tester won't catch. Step 09 shows why.

Steps 04 and 05 prepare the end of the cable. Here are both, in three pictures:

1. Boot on, if you're using one.
2. Strip about 1 in (25 mm) of jacket. Put the cable in the rotary stripper, turn it once, then flex the cable to snap the scored jacket and slide it off. If the blade is adjustable, set it to score the jacket, not cut through it.
3. Look at the eight wires. Any nicked insulation or bare copper means cutting the end off and starting again. A nick that looks harmless is where the cable fails later.
4. Cut off the spline, the plastic cross down the middle of most Cat6 cable, flush with the jacket. Cut off the thin rip cord too, if there is one.

Stripping more than an inch is fine at this stage, since you'll trim to length in Step 05. Stripping too little makes the wires hard to arrange.

1. Fan out the four pairs: orange, green, blue, brown.
2. Untwist each pair only back to the jacket, and no further.
3. Straighten every wire. Pull it between your thumb and finger, or bend it gently back and forth. Kinked wires won't slide into the plug's channels.
4. Lay them side by side in T568B order, white/orange first, and flatten them into a flat ribbon. Pinch it between thumb and finger and check the order once more against Step 03.
5. Trim straight across, all eight at once. For a standard plug, leave about ½ in (13 mm) beyond the jacket, and hold the plug alongside to check that the jacket will reach inside it. For a pass-through plug, just even up the ends and leave them long; the crimper trims them later. With a load bar, thread the wires through it in order, slide it down to the jacket, and trim to the length on the plug's packet.

This is what you're aiming for, seen from the side:

1. Hold the plug clip-down, gold contacts up, the way Step 03 showed.
2. Push the ribbon straight in, each wire into its own channel, firmly, until all eight touch the front wall (standard plug) or come out of the front (pass-through).
3. Look at the tip, end-on. You should see eight copper ends, all the way forward. One short wire is the most common reason a home-made cable fails.
4. Check the back. The jacket must be inside the plug, past the strain-relief wedge: the plastic bump at the back that the crimper presses down. If it isn't, pull the wires out, trim them a little shorter and try again.
5. Read the colours through the clear plastic, pin 1 to pin 8, against Step 03. After the crimp it's permanent, so this is your last chance.

1. Push the plug into the crimper's 8P8C (RJ45) slot until it seats, holding the cable so the wires can't back out.
2. Squeeze firmly, all the way. A ratchet crimper won't open again until it has done a full stroke. That one squeeze pushes all eight gold contacts down through the insulation into the copper, and drives the wedge onto the jacket.
3. Pass-through plug: the blade trims the ends. Check they're flush with the face of the plug, and snip off any leftover whisker with flush cutters. A single stray strand touching its neighbour is a short.
4. Look along the contacts from the side. All eight should be pushed down level. One sitting higher than the rest didn't bite: put it back in the crimper and squeeze again.
5. Tug the cable gently. The plug shouldn't move. Slide the boot over it.
6. Make the other end exactly the same way, in the same order: T568B again.

Plug one end into the tester's main unit and the other into the remote, then switch it on. The main unit sends a signal down each wire in turn, and the matching light on the remote should come on at the same moment: 1 with 1, 2 with 2, all the way to 8. Here's how to read anything else:

Then try it for real

Plug it between a computer and a switch or router. The port lights at both ends should come on, and the link should come up at full speed. Here's how to check the speed:

Get-NetAdapter | Select-Object Name, LinkSpeed

ifconfig en0 | grep media

On a Mac the built-in Ethernet port is usually en0. A USB or Thunderbolt adapter gets another number, and networksetup -listallhardwareports lists them all. On Linux, find the interface name with ip link, then cat /sys/class/net/enp3s0/speed prints 1000 for gigabit.

A basic tester checks one thing: does pin 3 at this end reach pin 3 at the other? It has no idea which wires are twisted together. So if you wire both ends in the “tidy” order, with each stripe followed by its own solid colour (white/orange, orange, white/green, green, white/blue, blue, white/brown, brown), every light comes on in step and the cable passes.

But Ethernet sends its signals on pins 1 & 2, 3 & 6, 4 & 5 and 7 & 8. In the tidy order, pins 3 and 6 are white/green and blue, two wires from different pairs, and so are pins 4 and 5. Those signals ride on wires that aren't twisted together, so noise and crosstalk stop cancelling.

The result is a cable that tests perfectly and then misbehaves. It might work over a short distance and fail over a long one, link at 100 Mb/s instead of gigabit, or drop packets under load. The only defences are wiring to T568B (or T568A) at both ends, or testing with a cable certifier, a much more expensive tester that measures crosstalk.

Both orders are in the cabling standard (TIA-568), and they perform identically: each puts every signal pair on one twisted pair. What matters is using the same one at both ends, and ideally the same one throughout a building. B is the usual choice for patch cables in North America. The standard itself prefers A for wall wiring, and US government work requires it. If the jacks in your building are already wired one way, match them.

A at one end and B at the other makes a crossover cable, which swaps the transmit and receive pairs. Older equipment needed one to connect two computers directly. Today almost every port works out the cable's direction for itself (it's called Auto-MDI-X), so you'll never need to make one. Just don't make one by accident.

For a cable run through walls, ceilings or between rooms, don't crimp plugs onto the cable itself. Use solid cable, end it in a keystone jack at each end (in a wall plate in the room, and in a patch panel or surface box near the router), and connect each device with a short factory-made patch cable.

Punching down a keystone jack

1. Strip about 1–2 in (25–50 mm) of jacket and cut off the spline, as in Step 04.
2. Find the colour labels on the jack. Each of its eight slots is colour-coded, usually in two rows marked A and B. Follow the B row, or A if that's what the rest of the building uses.
3. Lay each wire into its slot, keeping each pair twisted right up to the jack. The ½-inch rule from Step 05 applies here too.
4. Seat each wire with a 110 punch-down tool, its cutting side facing the outside of the jack. One push seats the wire and trims its end. (Some “tool-less” jacks have a cap that presses all eight wires down instead.)
5. Snap on the cap, if there is one. Then test the whole run with a patch cord plugged into each end.

Running the cable

• 100 m (328 ft) per link, end to end: up to 90 m of solid cable, plus no more than 10 m of patch cords at the two ends combined.
• Bend it gently. No tighter than about four times the cable's own diameter, with no kinks, no cable ties pulled tight, and no staples pinching it.
• Keep it away from mains power cables, and cross them at right angles where you have to.
• Check the jacket rating. Where local building code requires it, use plenum-rated (CMP) cable in air-handling spaces and riser-rated (CMR) cable between floors.

• You can say the T568B order from memory, and which end of the plug is pin 1
• Boots go on before the crimp
• No more than ½ in (13 mm) of untwisted wire, and the jacket inside the plug
• All eight copper ends at the front of the plug before you crimp
• Lights 1 to 8, in step, at both ends
• You can explain why a split pair passes a basic tester and still fails
• Solid cable in walls into keystone jacks; stranded patch cords to devices

Where to go next