What is the difference between T568A and T568B?

Only the orange and green pairs swap: in T568A, green is on pins 1-2 and orange on 3-6; in T568B, it is the other way round. Performance is identical. You just need the same standard at both ends of a straight cable.

Which pins does 100 Mbit/s Ethernet use?

10BASE-T and 100BASE-TX use only two pairs: pins 1 and 2 to transmit, 3 and 6 to receive, station side. Pins 4, 5, 7 and 8 stay free, which leaves room for PoE alternative B.

Does Gigabit Ethernet need all 8 wires?

Yes. 1000BASE-T transmits and receives at the same time on all four pairs. A two-pair cable, or one broken pair, drops the link to 100 Mbit/s when both interfaces allow it.

Do you still need crossover cables?

Rarely. Auto-MDIX interfaces detect the wiring and swap transmit and receive by themselves. A crossover cable only matters with old 10/100 equipment without Auto-MDIX connected peer to peer.

Pinout

RJ45 Ethernet pinout: T568A, T568B and pairs

The RJ45 connector on an Ethernet network (8P8C) has 8 contacts, wired as 4 twisted pairs. The T568A and T568B standards differ only by swapping the orange and green pairs. 10/100BASE-TX uses two pairs (pins 1-2 and 3-6); 1000BASE-T uses all four.

By Hugues Orgitello · Updated October 14, 2026 ·

RJ45, 8 contacts: colors, pairs and uses

1000BASE-T signal names per IEEE 802.3; 10/100BASE-TX in its own column, station side (MDI). Wire colors per ANSI/TIA-568.

Plug seen from the contact side, latch at the back, tip pointing up

Pin	Signal	T568A	T568B	10/100BASE-TX	PoE	Description
1	BI_DA+	White/green	White/orange	TX+	Alt. A	Pair A, positive (Differential pair)
2	BI_DA-	Green	Orange	TX-	Alt. A	Pair A, negative (Differential pair)
3	BI_DB+	White/orange	White/green	RX+	Alt. A	Pair B, positive (Differential pair)
4	BI_DC+	Blue	Blue	-	Alt. B (+)	Pair C, positive (Differential pair)
5	BI_DC-	White/blue	White/blue	-	Alt. B (+)	Pair C, negative (Differential pair)
6	BI_DB-	Orange	Green	RX-	Alt. A	Pair B, negative (Differential pair)
7	BI_DD+	White/brown	White/brown	-	Alt. B (-)	Pair D, positive (Differential pair)
8	BI_DD-	Brown	Brown	-	Alt. B (-)	Pair D, negative (Differential pair)

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T568A or T568B: which one?

Electrically, they are equivalent: only the colors of pairs 2 and 3 swap places. What matters is using the same standard at both ends of a straight cable. A cable with T568A at one end and T568B at the other is a crossover cable.

Crossover cables used to connect two devices of the same type (two PCs, two switches). Modern interfaces do Auto-MDIX and swap transmit and receive themselves, so a straight cable almost always works. Almost every 1000BASE-T PHY implements it.

PoE: which pairs carry power?

IEEE 802.3af and 802.3at (PoE and PoE+) deliver power over two pairs. In alternative A, the voltage rides in common mode on data pairs 1-2 and 3-6. In alternative B, it uses pairs 4-5 (positive) and 7-8 (negative), which 10/100BASE-TX leaves unused. A powered device (PD) must accept both, and both polarities, hence the diode or active bridge at its input.

IEEE 802.3bt (PoE++) adds four-pair power: optional for Type 3 (60 W at the source), mandatory for Type 4 (90 W at the source, 71.3 W available at the device). The polarities in the table are those of 802.3af and 802.3at; 802.3bt allows either polarity on alternative B.

On the board: magnetics, termination and shield

The PHY never connects straight to the RJ45: an isolation transformer (the “magnetics”) sits in between, often built into the connector (RJ45 MagJack). The cable-side center taps go through the so-called Bob Smith termination, a 75 Ω resistor per pair into a shared high-voltage capacitor to chassis.

Each pair is routed at 100 Ω differential. The RJ45 shielded shell goes to chassis ground, and the digital ground plane stays out from under the cable side of the magnetics: that is the isolation barrier.

Frequently asked questions

FAQ

What is the difference between T568A and T568B?: Only the orange and green pairs swap: in T568A, green is on pins 1-2 and orange on 3-6; in T568B, it is the other way round. Performance is identical. You just need the same standard at both ends of a straight cable.
Which pins does 100 Mbit/s Ethernet use?: 10BASE-T and 100BASE-TX use only two pairs: pins 1 and 2 to transmit, 3 and 6 to receive, station side. Pins 4, 5, 7 and 8 stay free, which leaves room for PoE alternative B.
Does Gigabit Ethernet need all 8 wires?: Yes. 1000BASE-T transmits and receives at the same time on all four pairs. A two-pair cable, or one broken pair, drops the link to 100 Mbit/s when both interfaces allow it.
Do you still need crossover cables?: Rarely. Auto-MDIX interfaces detect the wiring and swap transmit and receive by themselves. A crossover cable only matters with old 10/100 equipment without Auto-MDIX connected peer to peer.

Sources

ANSI/TIA-568.2-D, Balanced Twisted-Pair Telecommunications Cabling (paid or members-only document)
IEEE 802.3, clauses 14 (10BASE-T), 25 (100BASE-TX), 40 (1000BASE-T), 33 and 145 (PoE)

Tables built from these documents and cross-checked against manufacturer datasheets. Always check the datasheet of the connector you actually use: some manufacturers number differently.