How to Wire an Ethernet Cable

Wiring your own ethernet cables is a useful skill if you run network cable through walls, build patch cables to exact lengths, or just want to stop paying retail prices for pre-made cables that are always six inches too short or six feet too long. The process is mechanical and repeatable once you understand the standard, and it does not require any special electrical knowledge -- just the right tools, a steady hand, and attention to the pin order.

In this guide, you will learn what an ethernet cable is, the difference between the T568A and T568B wiring standards, when you need a straight-through cable versus a crossover, how Cat5e compares to Cat6, and a step-by-step procedure for crimping an RJ45 connector onto raw cable. We also cover testing, a pinout reference, Power over Ethernet wiring, and the mistakes that cause the most support calls.

What Is an Ethernet Cable?

An ethernet cable is a length of twisted-pair copper cable terminated at each end with a modular connector, commonly called an RJ45 connector. The correct technical name is an 8P8C connector -- 8 positions, 8 contacts -- but "RJ45" is the name everyone uses, including manufacturers, so this guide uses it too.

Inside the jacket are four pairs of copper conductors, each pair twisted at a different rate. The twisting is not cosmetic -- it cancels electromagnetic interference between adjacent pairs, which lets the cable carry a clean data signal over distance. Ethernet cable is used for wired network connections: computer to router, switch to patch panel, security camera to network video recorder, or any device with a network jack instead of Wi-Fi.

T568A vs T568B Wiring Standards

Almost all confusion around ethernet wiring comes down to one thing: there are two accepted wiring standards for which color wire goes into which pin, T568A and T568B. Both are electrically valid -- the pairs carry the same signals in the same roles, and the standards just swap which pair sits in which pin position. Pick one standard and use it consistently on both ends of a cable, and ideally throughout a building, so every cable behaves the same way.

T568A Pinout

T568B Pinout

T568B is the more common standard in US residential and commercial cabling today, and it is what most pre-made patch cables and electricians default to unless told otherwise. T568A is the standard required for US federal government installations, and it also matches the older USOC wiring used on legacy telephone systems, which made it the natural choice for backward compatibility during the transition from phone wiring to data wiring. Neither standard is "more correct" for performance -- consistency is what matters.

Straight-Through vs Crossover Cables

Once you know which standard you are using, decide whether you need a straight-through cable or a crossover cable.

A straight-through cable uses the same standard on both ends -- T568B on both ends, or T568A on both ends. This is what you need for the vast majority of connections: computer to switch, computer to router, switch to patch panel, camera to network video recorder. Any time you connect two dissimilar devices, straight-through is what you want, and it is what nearly every pre-made cable is built as.

A crossover cable uses T568A on one end and T568B on the other. Historically, a crossover cable was needed to connect two similar devices directly without a switch in between -- computer to computer, or switch to switch -- because it swaps the transmit and receive pairs so one device's transmit pins line up with the other device's receive pins.

Crossover cables are largely unnecessary today. Modern network interface cards and switches almost universally support Auto-MDIX, which automatically detects whether the link needs a crossover and corrects for it internally, regardless of which cable type you plug in. Unless you are working with older legacy hardware, wire every cable as straight-through and skip crossovers entirely.

Cat5e vs Cat6 vs Cat6a: Which Cable Do You Need?

The wiring procedure is the same regardless of cable category, but the raw cable you buy determines what speeds and distances the finished cable supports.

Cat5e is rated for Gigabit ethernet at the standard 100 meter maximum segment length and has a 100MHz bandwidth rating -- adequate for most home and small office wiring where 1Gbps is the ceiling anyway. Cat6 raises the bandwidth rating to 250MHz and can carry 10 Gigabit ethernet, but only over a shorter run of roughly 37 to 55 meters depending on installation conditions and crosstalk from neighboring cables; run Cat6 at the full 100 meters and it still delivers a reliable 1Gbps, just not 10Gbps. Cat6 is physically thicker and less flexible than Cat5e because of the tighter twists and the internal spline many manufacturers add to reduce crosstalk. For guaranteed 10 Gigabit performance over a full 100 meter run, step up to Cat6a.

Tools and Materials Needed

Gather the right tools before you start. Using the wrong connector type is one of the most common reasons a freshly crimped cable fails testing.

Tools

• RJ45 crimping tool (an 8P8C modular crimper, ideally with an integrated cutter and stripper)
• Cable stripper (or the stripping blade built into most crimping tools)
• Wire cutters or flush cutters
• A cable tester that checks continuity on all 8 pins

Materials

• Bulk ethernet cable (Cat5e, Cat6, or Cat6a, matched to your speed/distance needs)
• RJ45 connectors matched to your cable type

The connector-matching detail matters more than people expect. Bulk cable comes in two conductor types: solid-core, meant for permanent in-wall runs and patch panels, and stranded, more flexible and used for short patch cables that get flexed and re-plugged repeatedly. RJ45 connectors have different internal contact teeth for each type -- solid-core connectors pierce and grip a single solid strand, while stranded connectors bite into multiple thin strands. Mixing them up often produces a connection that passes a casual continuity check but fails intermittently under flexing. Buy connectors labeled for the cable type you are terminating.

Step-by-Step RJ45 Crimping Procedure

Follow these steps identically on both ends if you are building a straight-through cable.

1. Strip the outer jacket. Using your cable stripper or the stripping notch on the crimping tool, remove about 1 to 1.5 inches of jacket from the end of the cable. Work carefully -- a stripper set too deep will nick the inner conductors, and a nicked conductor is a weak point that can fail later.

2. Untwist and arrange the conductors. Untwist the four pairs and straighten the eight conductors. Fan them out flat, side by side, in the exact pin order for T568A or T568B, whichever standard you are using.

3. Trim the ends flush. Trim all eight conductors so the ends are even. An uneven trim means some conductors will not reach the front of the connector while others do.

4. Keep the untwisted length short. Check that the untwisted portion of each pair is under about 0.5 inch (13mm) before inserting the wires. Twisting cancels crosstalk, so every bit of untwisted wire is a small gap in that protection -- this matters more on Cat6 and Cat6a runs, but it is good practice on every cable.

5. Insert the conductors into the connector. Slide the flat, ordered conductors into the RJ45 connector so all eight reach the front and make contact with the gold pins, and so the outer jacket is captured under the connector's strain-relief clamp at the back. If the jacket is not seated under the clamp, the cable will pull loose from the crimp over time.

6. Crimp the connector. Insert the loaded connector into the crimping tool and squeeze firmly until you feel or hear it seat -- this drives the gold contacts through the insulation of each conductor and locks the strain-relief clamp onto the jacket. A proper crimp cannot be undone; the connector is single-use.

7. Repeat on the other end. For a straight-through cable, wire the second end to the exact same standard as the first -- both T568A, or both T568B. Mixing standards on the two ends produces a crossover cable by accident, a common source of "the cable is bad" support calls when a crossover was never intended.

Testing Your Ethernet Cable

Never trust a freshly crimped cable without testing it. Plug both ends into a cable tester and check for four things:

• Continuity on all 8 pins and all 4 pairs, confirming every conductor makes it from one end to the other.
• Shorts, where two conductors touch and read as connected when they should not be.
• Opens, where a conductor shows no connection, usually from a conductor that did not reach the front of the connector before crimping.
• Split pairs and reversed pairs, where the signal is continuous end to end but routed through the wrong physical pair or in the wrong order.

On a correctly wired straight-through cable, the tester should show pins lighting up in the exact 1-2-3-4-5-6-7-8 sequence on both ends, with no gaps and no pins out of order. On an intentional crossover cable, expect the transmit and receive pairs to show up swapped -- that is correct behavior, not a fault.

Pinout Reference: Pairs and Speed Requirements

The eight conductors are organized into four twisted pairs, and which pins carry which pair differs by standard.

This matters for troubleshooting because not every speed uses every pair. 10/100Mbps ethernet (10BASE-T and 100BASE-TX) only uses pins 1, 2, 3, and 6 -- Pair 2 and Pair 3. Pins 4, 5, 7, and 8 sit idle at those speeds. Gigabit ethernet and faster (1000BASE-T and above) uses all four pairs simultaneously, which is why a fault on pins 4, 5, 7, or 8 can look fine at 100Mbps and then fail at gigabit.

Power over Ethernet (PoE) Wiring Notes

PoE delivers DC power to a connected device over the same cable that carries the data signal, which is how PoE security cameras, access points, and VoIP phones run on a single cable with no separate power adapter. There are two ways PoE can be wired:

• Mode A (Alternative A) sends power over the same pairs used for 10/100 data -- pins 1, 2, 3, and 6.
• Mode B (Alternative A) sends power over the spare pairs otherwise unused on 10/100 links -- pins 4, 5, 7, and 8.

Common PoE standards are 802.3af (PoE, roughly 15.4W), 802.3at (PoE+, roughly 30W), and 802.3bt (PoE++, higher wattage for devices like PTZ cameras). A cable wired correctly to either T568A or T568B with all 8 conductors terminated supports PoE without special wiring changes -- PoE does not require a different pinout, but it does require all four pairs connected end to end. Do not leave a pair unterminated on a PoE run, since gigabit PoE and Mode B power both depend on pairs a sloppy 10/100-only crimp might leave disconnected.

Common Mistakes to Avoid

• Split pairs. Using two conductors from different twisted pairs to carry one signal pair. This is the most deceptive mistake because a basic continuity tester shows the cable as good -- current flows end to end -- but the signal loses its crosstalk cancellation and performs poorly under load. A tester that checks for split pairs will catch this; a simple continuity light will not.
• Untwisting too much wire before the connector. Every fraction of an inch of untwisted conductor is a gap in crosstalk protection. Keep it under 0.5 inch as described above.
• Accidentally mixing T568A and T568B ends. This turns a straight-through cable into a crossover, which will not link correctly with equipment expecting straight-through (Auto-MDIX will often save the connection, but not always).
• Not fully seating the conductors before crimping. An unseated conductor produces an open pin and an intermittent or dead connection.
• Mismatching connector and cable type. Solid-core connectors on stranded patch cable, or vice versa, as covered above.
• Exceeding 100 meters on a single segment. Ethernet over twisted-pair copper is rated for 100 meters end to end, including patch cables. Longer runs need a switch, media converter, or fiber segment in the middle.

Troubleshooting Table

Key Takeaways

• An ethernet cable is twisted-pair copper wire terminated in RJ45 (8P8C) connectors, used for wired network connections.
• T568A and T568B are both valid standards -- pick one and use it consistently on every cable and jack in a building.
• Straight-through cables (same standard both ends) connect dissimilar devices; crossover cables (T568A one end, T568B the other) are largely obsolete thanks to Auto-MDIX.
• Cat5e handles Gigabit at 100 meters; Cat6 handles 10Gbps only up to about 37-55 meters but 1Gbps at the full 100 meters; Cat6a delivers 10Gbps at 100 meters.
• Match RJ45 connectors to cable type -- solid-core and stranded cable need different connector contacts.
• Keep untwisted conductor length under 0.5 inch and test every finished cable on all 8 pins before relying on it.
• 10/100Mbps only uses pins 1, 2, 3, and 6; Gigabit and above uses all four pairs, which is why gigabit-specific faults point to Pair 1 or Pair 4.
• PoE works over a correctly wired standard cable with all 8 conductors terminated -- do not leave pairs unterminated on a PoE run.
• Split pairs are the most deceptive mistake because they often pass a basic continuity check while still degrading performance.

Frequently asked questions

What is the difference between T568A and T568B?

Both are valid ethernet wiring standards that assign the same four pairs to the same pin roles, but swap which pair sits in pins 1-2 and 3-6. T568B is more common in US residential and commercial cabling; T568A is the US federal standard and matches older phone wiring. Use one standard consistently on both ends of every cable.

Do I need a crossover cable to connect two computers?

Not anymore. Older network cards required a crossover cable (T568A on one end, T568B on the other) to connect two similar devices directly. Modern network interface cards and switches support Auto-MDIX, which automatically detects and corrects the pair orientation, so a standard straight-through cable works for nearly all connections today.

Is Cat6 always better than Cat5e?

Cat6 supports higher bandwidth (250MHz vs 100MHz) and can carry 10 Gigabit ethernet, but only over about 37-55 meters; at the full 100 meter range it delivers the same reliable 1Gbps as Cat5e. If you only need Gigabit speeds, Cat5e is adequate. For guaranteed 10Gbps at 100 meters, use Cat6a instead.

Why does my ethernet cable test fine but the connection is slow?

A common cause is a split pair -- two conductors from different twisted pairs wired into one signal pair. A basic continuity tester shows the cable as good because current still flows end to end, but the signal loses its crosstalk cancellation, degrading performance. A tester that specifically checks for split pairs will catch this issue.

Can I wire ethernet cable for Power over Ethernet differently than normal?

No special wiring is needed. A cable wired correctly to either T568A or T568B with all 8 conductors terminated supports PoE, since PoE rides on the same pairs as data (Mode A) or the spare pairs (Mode B). The only requirement is that all four pairs are actually connected end to end -- do not leave pairs unterminated on a PoE run.

What connector do I need for solid-core versus stranded ethernet cable?

RJ45 connectors are built with different internal contact teeth for each conductor type. Solid-core connectors have teeth designed to pierce and grip a single solid strand, while stranded connectors bite into multiple thin strands. Mixing them up often passes a casual continuity check but fails intermittently under flexing, so match the connector to the cable type.

Interactive diagrams for this guide

• Cat5 Diagram
• Cat 5 Termination Diagram
• Poe Wiring Diagram
• Crossover Wiring Diagram
• Crossover Circuit Diagram
• Switch Diagram In Networking
• Cctv Network Diagram
• Rj11 Phone Jack Wiring Diagram

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