RJ45 Pinout Diagram & Wiring Guide: T568A vs T568B (2026)

The RJ45 pinout is one of the most searched topics in networking, with "RJ45 pinout" receiving approximately 8,100 Google searches per month. Whether you are crimping a patch cable, wiring a wall plate, or designing a PCB with an ICM RJ45 connector, understanding the correct pin assignment is essential.

This guide covers the two RJ45 wiring standards, T568A and T568B, their pin assignments, when to use each, and common wiring mistakes to avoid. We also cover the pinout for different Ethernet speeds (10/100Base-T vs 1000Base-T) and PoE wiring.

T568A vs T568B: The Two Wiring Standards

There are two recognized wiring standards for RJ45 connectors, defined by TIA/EIA-568:

• T568A: Defined by TIA/EIA-568-A (1991). The older standard, still required by some government and military contracts. In T568A, Pair 1 (blue) is on pins 4-5, Pair 2 (orange) is on pins 3-6, Pair 3 (green) is on pins 1-2, and Pair 4 (brown) is on pins 7-8.
• T568B: Defined by TIA/EIA-568-B (2001). The more common standard, used in most commercial and residential installations. In T568B, Pair 1 (blue) is on pins 4-5, Pair 2 (green) is on pins 3-6, Pair 3 (orange) is on pins 1-2, and Pair 4 (brown) is on pins 7-8. The only difference from T568A is that the orange and green pairs are swapped.

Complete RJ45 Pinout Tables

T568B Pinout (Most Common)

T568A Pinout

Side-by-Side Comparison

Which Standard Should You Use?

General Rule

Use T568B for commercial and residential installations (it is the more common standard in the US and most of the world). Use T568A if required by contract specifications (some government and military projects mandate T568A). The critical rule is consistency: both ends of a straight-through cable must use the same standard.

Straight-Through vs Crossover Cables

• Straight-through cable: Both ends use the same standard (e.g., T568B on both ends). Used for connecting different device types (PC to switch, switch to router). This is the standard patch cable used in 99% of installations.
• Crossover cable: One end uses T568A, the other uses T568B. Used for connecting same-type devices (PC to PC, switch to switch). Note: Modern Gigabit Ethernet and above use auto-MDIX, which automatically detects and adjusts for straight-through or crossover cables, making crossover cables largely obsolete.

Auto-MDIX: Why Crossover Cables Are Obsolete

Auto-MDIX (Automatic Medium-Dependent Interface Crossover) is a feature introduced in the Gigabit Ethernet standard (1000Base-T) that automatically detects whether the connected port needs a straight-through or crossover connection and adjusts accordingly. Since 1000Base-T uses all 4 pairs bidirectionally, the PHY can dynamically assign transmit and receive pairs. This means you can use a straight-through cable for all connections, regardless of device type.

RJ45 Pinout by Ethernet Speed

PoE Pinout: How Power Is Delivered Over RJ45

Power over Ethernet (PoE) delivers DC power over the same RJ45 cable that carries data. There are two modes defined by IEEE 802.3:

Mode A (Phantom Power)

Power is delivered on the same pins as data (pins 1-2 and 3-6). The DC voltage is superimposed on the data signal using center-tapped transformers. This is the most common mode for 802.3af and 802.3at.

Mode B (Spare Pair Power)

Power is delivered on the unused pairs (pins 4-5 positive, pins 7-8 negative). In 10/100Base-T, these pairs are unused for data, making Mode B simple. In Gigabit Ethernet, all pairs carry data, so Mode B also uses center-tapped transformers.

PoE++ (802.3bt Type 3 and 4)

PoE++ uses both Mode A and Mode B simultaneously to deliver up to 90W. All 4 pairs carry both data and power. The center tap of each transformer carries the DC current. This requires ICM RJ45 connectors with magnetics rated for the full PoE current.

Common RJ45 Wiring Mistakes

Mistake 1: Mixing T568A and T568B on a Straight-Through Cable

If one end is T568A and the other is T568B, you have accidentally created a crossover cable. On older 10/100Base-T equipment without auto-MDIX, this will prevent link establishment. Always verify both ends use the same standard for straight-through cables.

Mistake 2: Splitting Pairs Across Non-Adjacent Pins

In 10/100Base-T, pins 3 and 6 must be the same pair (green or orange). A common mistake is to wire pins 3, 4, 5, 6 sequentially with 4 different wires, splitting the pair. This causes excessive crosstalk and can prevent the link from working at 100 Mbps.

Mistake 3: Untwisting Too Much Cable

Twisted pairs rely on the twist to cancel electromagnetic interference. If you untwist more than 13mm (0.5 inch) of wire at the connector, the crosstalk performance degrades significantly, especially at Gigabit speeds. Keep the twist as close to the connector as possible.

Mistake 4: Using the Wrong Cable Category

A Cat5e connector on Cat6 cable (or vice versa) creates an impedance mismatch. The wire gauge and twist rate differ between categories, so always match the connector to the cable category. Using a Cat5e connector on a Cat6 cable will limit performance to Cat5e levels.

VITALCONN ICM RJ45: Pre-Wired for Compliance

When you use VITALCONN's ICM (Integrated Connector with Magnetics) RJ45 connectors, the pinout and wiring are already handled internally. The magnetic module is pre-wired to the correct pins according to IEEE 802.3 standards, ensuring compliance and eliminating wiring errors. You only need to connect the PCB traces to the connector's through-hole or SMT pads according to the datasheet pinout.

Advantages of using ICM RJ45 instead of discrete magnetics:

• No risk of wiring mistakes — the magnetics are factory-tested and pre-wired
• Guaranteed IEEE 802.3 compliance for insertion loss, return loss, and crosstalk
• Simplified PCB layout — no need to place and route discrete transformers
• PoE center taps are pre-configured for Mode A and/or Mode B
• EMI shielding is integrated into the connector housing

VITALCONN offers 163+ ICM RJ45 models covering T568A/T568B compatible pinouts, with speeds from 10/100M to 10G and PoE support up to 90W. Our ARP platform also provides 26 cross-reference replacements for Pulse, BelFuse, TE, and Molex RJ45 connectors.

Frequently Asked Questions (FAQ)