RJ45 Pinouts – T568A vs T568B, Straight-Through, Crossover & PoE Wiring

1. The RJ45 Connector

The RJ45 connector (Registered Jack 45) is the 8-position, 8-contact (8P8C) modular plug used to terminate twisted-pair Ethernet cables. It is the physical connector for all copper Ethernet standards from 10BASE-T through 10GBASE-T, and is used on network interface cards, switches, routers, patch panels, wall jacks, and IP phones.

2. Twisted Pair Structure — Why Pairs Matter

A Cat5e/Cat6 cable contains four twisted pairs (8 wires total). The twisting of each pair cancels electromagnetic interference (EMI) through differential signalling and reduces crosstalk between adjacent pairs. Each pair carries one differential signal: one wire carries the positive (+) signal and the other carries the negative (−) complement.

3. T568A Wiring Standard — Full Pinout

T568A is defined in ANSI/TIA-568 and is the preferred standard for new U.S. government and federal installations, and is also the international preference per ISO/IEC 11801.

Pin	T568A Colour	Pair	Signal (100BASE-TX)	Signal (1000BASE-T)
1	White/Green	Pair 3 (+)	TX+ (Transmit positive)	BI_DA+ (Bidirectional A+)
2	Green	Pair 3 (−)	TX− (Transmit negative)	BI_DA− (Bidirectional A−)
3	White/Orange	Pair 2 (+)	RX+ (Receive positive)	BI_DB+ (Bidirectional B+)
4	Blue	Pair 1 (+)	Unused (100BASE-TX)	BI_DC+ (Bidirectional C+)
5	White/Blue	Pair 1 (−)	Unused (100BASE-TX)	BI_DC− (Bidirectional C−)
6	Orange	Pair 2 (−)	RX− (Receive negative)	BI_DB− (Bidirectional B−)
7	White/Brown	Pair 4 (+)	Unused (100BASE-TX)	BI_DD+ (Bidirectional D+)
8	Brown	Pair 4 (−)	Unused (100BASE-TX)	BI_DD− (Bidirectional D−)

Memory aid for T568A (pins 1-8): WG - G - WO - B - WB - O - WBr - Br
(White/Green, Green, White/Orange, Blue, White/Blue, Orange, White/Brown, Brown)

4. T568B Wiring Standard — Full Pinout

T568B is more common in commercial and legacy North American installations. Both T568A and T568B are electrically equivalent — the only difference is the position of the orange and green pairs (pins 1,2 and 3,6).

Pin	T568B Colour	Pair	Signal (100BASE-TX)	Signal (1000BASE-T)
1	White/Orange	Pair 2 (+)	TX+ (Transmit positive)	BI_DA+
2	Orange	Pair 2 (−)	TX− (Transmit negative)	BI_DA−
3	White/Green	Pair 3 (+)	RX+ (Receive positive)	BI_DB+
4	Blue	Pair 1 (+)	Unused (100BASE-TX)	BI_DC+
5	White/Blue	Pair 1 (−)	Unused (100BASE-TX)	BI_DC−
6	Green	Pair 3 (−)	RX− (Receive negative)	BI_DB−
7	White/Brown	Pair 4 (+)	Unused (100BASE-TX)	BI_DD+
8	Brown	Pair 4 (−)	Unused (100BASE-TX)	BI_DD−

Memory aid for T568B (pins 1-8): WO - O - WG - B - WB - G - WBr - Br
(White/Orange, Orange, White/Green, Blue, White/Blue, Green, White/Brown, Brown)

5. T568A vs T568B — Side-by-Side Comparison

Pin	T568A Colour	T568B Colour	Same?
1	White/Green	White/Orange	Different
2	Green	Orange	Different
3	White/Orange	White/Green	Different
4	Blue	Blue	Same
5	White/Blue	White/Blue	Same
6	Orange	Green	Different
7	White/Brown	White/Brown	Same
8	Brown	Brown	Same

Only four pins differ — 1, 2, 3, and 6. The green and orange pairs simply swap positions. Pins 4, 5, 7, and 8 (blue and brown pairs) are identical in both standards. Both are electrically equivalent — neither is faster or better. Consistency throughout an installation is what matters.

Criteria	T568A	T568B
Global preference	International standard (ISO/IEC 11801)	North American commercial preference
U.S. Government	Required for federal installations (per TIA-568)	Not specified for federal use
Legacy compatibility	Compatible with older USOC voice wiring	Most existing N. American installations use T568B
New deployments	Preferred for greenfield installations	Acceptable; widely used in commercial environments
Crossover cable	One end of a crossover cable is T568A	Other end of a crossover cable is T568B

6. Straight-Through, Crossover, and Rollover Cables

Cable Type	Wiring	Use Cases	Why It Works
Straight-Through	Both ends identical (T568B–T568B or T568A–T568A)	PC/server to switch, PC/server to router, switch to router, IP phone to switch, AP to switch	TX pins (1,2) of the DTE connect to RX pins (1,2) of the switch; the switch internally crosses pairs — the switch handles the signal crossing internally
Crossover	One end T568A, other end T568B	PC to PC (direct), switch to switch (no uplink port), router to router, hub to hub — connecting two like devices	TX pins (1,2) of Device A connect directly to RX pins (3,6) of Device B — the cable itself performs the crossing since neither device crosses internally
Rollover (Console)	Complete pin reversal (pin 1 connects to pin 8, etc.)	PC/laptop to Cisco router/switch console port via RJ45 (with USB-to-serial or DB-9 adapter)	Not an Ethernet cable — carries RS-232 serial signals for out-of-band management; connects to console port only, not Ethernet ports
Loopback	TX pins (1,2) wired to RX pins (3,6) within the same plug	Testing NIC transmit/receive circuitry, port diagnostics	Loops transmitted signal back into receiver — used by diagnostic software to test the interface hardware

Auto-MDIX: Most switches and NICs manufactured after ~2001 support Auto-MDIX (Automatic Medium-Dependent Interface Crossover), which automatically detects whether a crossover or straight-through cable is needed and adjusts the internal circuitry accordingly. For the CCNA exam, always know the manual cable selection rules regardless of Auto-MDIX.

7. 100BASE-TX vs 1000BASE-T Pin Usage

The number of active pairs differs significantly between Fast Ethernet and Gigabit Ethernet — this has important implications for cabling faults and PoE deployment.

8. PoE — Power over Ethernet and Pin Pairs

PoE delivers DC power to connected devices through the same RJ45 cable as data. Understanding which pin pairs carry power matters for troubleshooting and ensuring cabling supports PoE. See also: Voice VLAN for IP phone port configuration.

PoE Standard	IEEE Standard	Max Power	Power Pairs Used	Notes
PoE	802.3af	15.4 W	Mode A: pins 1,2 and 3,6 Mode B: pins 4,5 and 7,8	DC voltage superimposed on data or spare pairs; Cat5e minimum
PoE+	802.3at	30 W	Same as 802.3af	Higher current on same pairs; Cat5e minimum
PoE++ Type 3	802.3bt	60 W	All four pairs (Mode A + Mode B simultaneously)	Requires all four pairs intact; Cat6 recommended
PoE++ Type 4	802.3bt	100 W	All four pairs	Cat6A required for long runs; powers thin clients, displays, high-load APs

9. Cable Categories — Choosing the Right Cable

Category	Max Bandwidth	Max Speed / Distance	Typical Use
Cat5e	100 MHz	1 Gbps / 100 m	Minimum for new installations; supports 802.3af/at PoE
Cat6	250 MHz	1 Gbps / 100 m; 10 Gbps / 55 m	Recommended for new deployments; better crosstalk rejection; supports PoE++
Cat6A	500 MHz	10 Gbps / 100 m	10GBASE-T at full 100 m; high-density PoE++ (100W); data centres
Cat7	600 MHz	10 Gbps / 100 m	Industrial / high-EMI environments; uses GG45/TERA connectors (not standard RJ45)
Cat8	2000 MHz	25–40 Gbps / 30 m	Data centre top-of-rack switch connections; short-run server links

See Ethernet Standards and Fiber vs Copper for speed/distance comparisons across all cable types and fibre options.

10. Step-by-Step Cable Termination

Tools required: Cable stripper, RJ45 crimping tool, RJ45 connectors (8P8C), cable tester.

11. Cable Faults — Types and Diagnosis

Fault Type	Definition	Cause	Tester Indication	Fix
Miswire	Wires connected to wrong pins at one or both ends	Incorrect colour order; mixed T568A/B on straight-through cable	Pins show out-of-sequence (e.g., pin 1 maps to pin 3)	Re-terminate faulty end in correct colour order
Open circuit	A wire has no continuity	Wire not fully inserted; broken wire; damaged pin	Pin absent from tester readout on remote end	Re-strip and re-terminate; replace cable if mid-run break
Short circuit	Two or more wires electrically connected	Damaged insulation; excessive strip damage; bent pin	Two pins show connected to same remote pin	Re-terminate; inspect insulation; replace if mid-cable
Split pair	Wires from different pairs used together on same position	Wiring pins 1 and 3 together instead of 1 and 2	Basic continuity PASSES; elevated NEXT on advanced tester; link works at 100 Mbps but fails Gigabit	Re-terminate keeping same-pair wires together; verify with NEXT-capable certifier
Reversed pair	Both wires of a pair connected but polarity swapped	White/colour wire and colour wire swapped within a pair	Tester shows pair reversed (e.g., pin 1 maps to pin 2 on remote)	Re-terminate; keep stripe wire before solid within each pair

See Cable Testing Tools for a full guide to cable testers, toners, and certifiers. Use show interfaces on Cisco IOS to check for input errors caused by physical cable faults.

12. Auto-MDIX — Modern Cable Flexibility

Auto-MDIX automatically detects whether a straight-through or crossover cable is connected and adjusts the port's internal circuitry. This eliminates cable-type confusion on modern equipment.

! Verify Auto-MDIX on Cisco IOS:
Switch# show interfaces GigabitEthernet0/1 | include MDIX
  Medium is copper, Auto-MDIX enabled

! Disable Auto-MDIX:
Switch(config-if)# no mdix auto

! Re-enable Auto-MDIX:
Switch(config-if)# mdix auto

13. Standards Compliance, Labelling, and Best Practices

Choose one standard per site: Never mix T568A and T568B randomly in the same installation — mixing creates accidental crossover cables.
Label every cable: Mark both ends with a unique identifier (e.g., "Rm101-PC1-SW-Gi1/0/5"). Labels prevent hours of troubleshooting.
Test after termination: Test every cable before installing in wall plates or raceways. Fixing in a bundle takes seconds; after installation takes hours.
Document the installation: Record port-to-device mappings, cable lengths, and test results. Required for enterprise installations.
Minimum bend radius: Do not kink Cat5e/Cat6 cables — minimum bend radius is 4× cable diameter. Kinks change twist geometry and degrade crosstalk performance.
Maximum cable run: 100 m total (90 m permanent link + 10 m patch cords). Exceeding this causes signal degradation and Gigabit negotiation failure.
Untwist minimally: Keep untwisted length under 13 mm inside the connector. Excessive untwisting is the leading cause of split pairs and NEXT failures on certified cabling.

14. Key Points & Exam Tips

RJ45 pin 1 is on the left when holding the connector with the clip down and contacts facing you.
T568A (pins 1-8): WG, G, WO, B, WB, O, WBr, Br. — T568B (pins 1-8): WO, O, WG, B, WB, G, WBr, Br. Only pins 1, 2, 3, 6 differ — green and orange pairs swap.
Straight-through: Both ends same standard → connects unlike devices (PC to switch, PC to router). Crossover: T568A one end, T568B the other → connects like devices (switch to switch, PC to PC).
Rollover (console) cable: Complete pin reversal (1↔8) → PC to Cisco console port; carries RS-232 serial, not Ethernet.
100BASE-TX uses only 2 pairs (pins 1,2 and 3,6). 1000BASE-T uses all 4 pairs — a cable with blue/brown pair faults works at 100 Mbps but fails Gigabit.
PoE 802.3af/at uses Mode A (data pairs) or Mode B (spare pairs). PoE++ 802.3bt (60W/100W) requires all four pairs; Cat6A for long runs.
Split pair passes basic continuity testing but causes crosstalk and Gigabit failure. Only a cable certifier with NEXT measurement will detect it.
Auto-MDIX allows modern devices to auto-detect cable type — know the manual rules for the CCNA exam.
Maximum run: 100 m. Minimum bend radius: 4× cable diameter. Cat6A for 10 Gbps at full 100 m runs.

15. RJ45 Pinouts and Wiring Standards Quiz

A network technician terminates a Cat6 cable for a PC-to-switch connection. She wires one end T568B and the other end also T568B. After plugging in, the link light does not come on. A cable tester shows all 8 pins mapping correctly in sequence. What is the most likely problem?

A. The cable needs to be a crossover — PC-to-switch always requires crossover cables B. T568B is the wrong standard — only T568A works for PC-to-switch connections C. Both ends being T568B is correct for a straight-through cable — the wiring is not the problem. Check other causes: is the switch port enabled? Is the NIC functioning? Is the connector fully seated in both ports? D. T568B requires shielded cable for PC-to-switch connections

Correct answer is C. A PC-to-switch connection requires a straight-through cable — both ends wired identically (T568B–T568B or T568A–T568A). The cable is correctly terminated. The cable tester confirming all 8 pins map correctly proves the cable is fine. The missing link light with a correct cable points to non-wiring causes: the switch port may be administratively shut (shutdown in Cisco IOS); the NIC may be disabled; the connector may not be fully clicked in; or there could be a hardware failure. T568A and T568B are equally valid for straight-through cables — the standard used does not affect functionality.

A cable tester shows all 8 pins with correct continuity, but the connection runs at only 100 Mbps instead of the expected 1 Gbps, despite both the switch and NIC supporting Gigabit. What wiring fault causes this specific symptom?

A. A miswire — pins mapped to wrong positions B. A split pair — wires from different pairs used together. Basic continuity passes because all wires connect, but 1000BASE-T's four-pair DSP fails because the pairs are not properly matched. 100BASE-TX only needs two pairs and is less sensitive to crosstalk C. An open circuit on pin 4 or 5 D. The cable is Cat4 and cannot support Gigabit

Correct answer is B. A split pair is the classic cause of this symptom. It occurs when wires from different physical pairs are used together on adjacent pins — for example, White/Orange (pair 2+) and White/Green (pair 3+) on pins 1 and 2. Basic continuity testers only verify that pin 1 on one end connects to pin 1 on the other — not which actual wire pair carries the signal. So split pairs pass basic testing. However, 1000BASE-T uses all four pairs with complex DSP echo cancellation that depends on proper pair twist rates. With a split pair, the wires have mismatched twist rates — DSP cancellation fails, causing severe NEXT. Gigabit negotiation fails and the link falls back to 100 Mbps. Only an advanced certifier measuring NEXT will reliably detect split pairs.

An engineer needs to connect two Cisco switches together. One is a Catalyst 2960 from 2005 (no Auto-MDIX) and the other is a Catalyst 9300 from 2020 (has Auto-MDIX). What cable should be used?

A. Straight-through cable — switches always use straight-through B. Either cable works — both switches have Auto-MDIX C. Rollover cable — this is a switch-to-switch management link D. A crossover cable is the correct choice (switch-to-switch = like devices). The 9300's Auto-MDIX will also make a straight-through cable work, but the 2960 without Auto-MDIX requires a crossover for guaranteed functionality

Correct answer is D. Two switches are like devices — both are MDI-X. Without Auto-MDIX, connecting two MDI-X devices with a straight-through cable puts TX on TX and RX on RX — neither device receives the other's transmissions. A crossover cable corrects this. The 2960 (2005) does not have Auto-MDIX and requires the correct cable. The 9300 (2020) has Auto-MDIX and can adapt to either cable type. Since the 2960 cannot adapt, a crossover cable is mandatory. A crossover works in either direction — it doesn't matter which end connects to which switch.

A network engineer needs console access to an unconfigured Cisco router. She has a USB-to-serial adapter on her laptop. What cable connects to the router's RJ45 console port, and why is it different from Ethernet cables?

A. A rollover (console) cable with complete pin reversal (pin 1 to pin 8, pin 2 to pin 7, etc.). It is not an Ethernet cable — it carries RS-232 serial signals for out-of-band management, not Ethernet frames. Used exclusively for console access, not data networking B. A straight-through cable — console ports are standard Ethernet ports C. A crossover cable — laptop and router are like devices D. A straight-through Cat6 cable — Cat6 is required for console speeds

Correct answer is A. The console (rollover) cable has complete pin reversal: pin 1 to pin 8, pin 2 to pin 7, pin 3 to pin 6, pin 4 to pin 5 (symmetrically). It carries RS-232 serial signals — not Ethernet. The console port is a serial port using an RJ45 connector for convenience. Terminal emulation software (PuTTY, Tera Term) connects at 9600 baud, 8N1 settings by default. This connection is used for initial device configuration before any IP address exists, password recovery, and troubleshooting when network access is unavailable. Cable category (Cat5/6) is irrelevant — serial signals run at 9600 baud regardless.

A PoE switch needs to power a high-end Wi-Fi 6E access point requiring 60W (802.3bt Type 3). The Cat6 cable has correct T568B continuity on all 8 pins. The AP keeps rebooting. What wiring issue could cause this?

A. T568B cannot support PoE — re-terminate to T568A B. Cat6 cannot carry PoE — upgrade to fibre C. 802.3bt (60W) requires all four pairs to carry power. If the blue and brown pairs (pins 4,5 and 7,8) have high resistance due to poor termination, Mode B power delivery fails. Data works fine on two pairs, but the additional power pairs must also be low-resistance for high-power PoE D. 60W PoE requires a specialised external PoE injector, not a PoE switch

Correct answer is C. 802.3bt Type 3 (60W) and Type 4 (100W) use both Mode A (data pairs, pins 1,2 and 3,6) and Mode B (spare pairs, pins 4,5 and 7,8) simultaneously to share the power load. Lower PoE standards (802.3af and 802.3at) only need two pairs. If pins 4,5 and 7,8 have high contact resistance due to poor termination or damage, Mode B power is compromised — the AP reboots because it cannot sustain its 60W budget. Data continues working fine since 100BASE-TX only uses two pairs. Fix: re-terminate the blue and brown pairs, verify resistance below specification, and test with a PoE-capable certifier. T568A vs T568B is irrelevant to PoE.

A cable passes the tester with correct pin order, the link light comes on, but data transfer is very slow with frequent drops. What fault is most likely present and how is it confirmed?

A. Cable length exceeds 100 m — basic testers don't check length B. A split pair — basic testers miss this. The link establishes because 100BASE-TX works on two pairs, but elevated NEXT causes CRC errors and retransmissions. Confirm with an advanced certifier measuring NEXT, attenuation, and return loss C. Open circuit on pin 4 — basic tester missed it D. The switch port has the wrong VLAN assigned

Correct answer is B. Link establishes + poor performance with drops is the hallmark of a split pair. A split pair places wires from different twisted pairs on adjacent pin positions. Since each pair has a different twist rate, differential signalling is compromised — severe NEXT (Near-End Crosstalk) results. At 10/100 Mbps the link sometimes maintains with high error rates; at 1 Gbps it typically fails entirely. A basic continuity tester only verifies pin-to-pin mapping — it doesn't measure which physical wire pair carries the signal. Only a cable certifier with NEXT measurement (Fluke DSX, Ideal SignalTEK) will confirm a split pair. A VLAN mismatch (option D) would cause no connectivity at Layer 2, not intermittent physical-layer drops.

What is the correct T568B wiring order for pins 1 through 8, left to right with the RJ45 clip down and contacts facing you?

A. WG, G, WO, B, WB, O, WBr, Br — this is T568A B. O, WO, G, B, WB, WG, Br, WBr C. WO, WG, O, B, WB, G, WBr, Br D. WO, O, WG, B, WB, G, WBr, Br — White/Orange, Orange, White/Green, Blue, White/Blue, Green, White/Brown, Brown

Correct answer is D. T568B pin order from left to right: White/Orange, Orange, White/Green, Blue, White/Blue, Green, White/Brown, Brown. Memory aid: "WO-O-WG-B-WB-G-WBr-Br". In T568B, Orange is first (pins 1,2) and Green is split across pins 3 and 6. Blue and Brown (pins 4,5 and 7,8) are identical in both T568A and T568B. Option A is T568A order where Green comes first (pins 1,2) and Orange is split (pins 3,6). This is the only difference between the two standards.

An installer crimps 50 Cat6 patch cables. After testing, 3 cables pass continuity but auto-negotiate at 100 Mbps instead of 1 Gbps. Cables are all under 10 metres. What should the installer do?

A. Use a cable certifier to test NEXT, attenuation, and split pairs on those 3 cables — basic continuity testers do not measure these parameters. Re-terminate any failing cables ensuring proper pair integrity and minimal untwisting B. Replace Cat6 with Cat6A — Cat6 cannot support Gigabit on short runs C. Configure speed 1000 on the switch ports — Gigabit does not auto-negotiate D. The cable tester confirms the cables are fine — check switch firmware instead

Correct answer is A. Cat6 absolutely supports Gigabit on short runs — 1000BASE-T is rated for 100 m on Cat5e. Speed auto-negotiation down to 100 Mbps on properly wired short Cat6 cables strongly suggests split pairs — passing basic continuity but defeating 1000BASE-T's four-pair DSP. A cable certifier (Fluke DSX, Ideal AnyWARE) measures NEXT, FEXT, attenuation, and return loss required by TIA-568. If NEXT fails, re-terminate keeping all pair wires together with minimal untwisting (under 13 mm). Forcing speed with speed 1000 without matching duplex causes duplex mismatch. The basic tester's "pass" result is not a reliable indicator of Gigabit suitability — that requires certification-level testing.

An engineer is deploying 802.3af PoE (15.4W) IP cameras on existing Cat5e T568B cabling. Cables pass continuity testing. What additional verification should he perform before deploying?

A. Nothing — Cat5e and T568B are PoE-compatible by definition B. Re-terminate to T568A — T568B is not PoE compatible C. Verify cable length does not exceed 100 m (PoE voltage drop increases with distance); test cable DC resistance per pair (should be below 25 Ohms per conductor per TIA-568 for PoE); and confirm all four pairs are terminated and intact D. Replace Cat5e with Cat6A — Cat5e cannot carry PoE

Correct answer is C. Cat5e is the minimum specified for PoE, and T568A vs T568B has absolutely no effect on PoE compatibility — both use the same four pairs. However, PoE-specific checks are important. Length matters: at 802.3af's 48V DC, a long Cat5e run has significant resistance — TIA/EIA TSB-184-A specifies maximum DC resistance of 25 Ohms per conductor for PoE reliability. Aged cabling may have increased resistance from oxidised connections. Cables near or exceeding 100 m may provide insufficient voltage at the camera. All four pairs being intact is also important as some PoE modes use the spare pairs. Cat5e is perfectly capable of carrying 802.3af and 802.3at PoE.

An engineer terminates 20 patch cables using T568B. A colleague says they should have used T568A because "T568B is being phased out." Is this correct, and does it matter for this installation?

A. The colleague is correct — T568B is obsolete and cables wired T568B will not work B. The colleague is partially correct — TIA-568 does prefer T568A for new and federal installations, but T568B is fully valid and widely used. Both are electrically identical. What matters is that the SAME standard is used consistently throughout — mixing standards creates accidental crossover cables C. The colleague is wrong — T568B is the only current standard; T568A was deprecated D. Neither standard matters — Auto-MDIX handles any wiring configuration

Correct answer is B. Both T568A and T568B are current valid ANSI/TIA-568 standards. T568B is not deprecated — it remains the most widely deployed standard in North American commercial installations. TIA-568 has a preference for T568A in new deployments and mandates it for U.S. federal government installations (largely for compatibility with legacy USOC voice wiring). ISO/IEC 11801 also prefers T568A internationally. For the vast majority of commercial installations both are equally valid. T568A and T568B are performance-wise and electrically identical — same four pairs, same PoE compatibility, same Gigabit support. The only problem mixing them creates: if one end of a straight-through cable is T568A and the other is T568B, the orange and green pair positions swap — creating an accidental crossover cable. Golden rule: pick one standard and use it everywhere — patch panels, wall jacks, and patch cables must all match.