RJ45

The RJ45 connector (technically 8P8C — 8 Position 8 Contact) is the rectangular plastic plug crimped onto the end of twisted-pair Ethernet cable. It has eight gold-plated pins that make contact with eight conductors inside the cable when inserted into an RJ45 port. The connector's locking tab clicks into the port and must be pressed to release. Despite being called "RJ45", the correct technical designation is 8P8C — true RJ45 is a slightly different connector used in telecom — but "RJ45" is universally understood in networking contexts.

Pin assignments: T568A vs T568B

Two TIA/EIA wiring standards define how the eight conductors map to the eight pins. T568B is the dominant standard in North America for new commercial and residential installations. T568A is the international standard (ISO/IEC 11801) and required for US government (TIA-568-C) work. The two standards differ only in which pairs occupy pins 1/2 and 3/6 — electrical performance is identical.

Gigabit Ethernet (1000BASE-T) uses all four pairs bidirectionally (BI_DA through BI_DD) simultaneously. Fast Ethernet (100BASE-TX) uses only pairs on pins 1/2 and 3/6. This is why Cat5e or better is required for Gigabit — Cat5 does not reliably support four-pair simultaneous signalling at 125 MHz.

Straight-through vs crossover cables and Auto-MDI-X

A straight-through cable uses the same wiring standard (T568A or T568B) on both ends — pin 1 connects to pin 1, pin 2 to pin 2, and so on. This is the standard patch cable used to connect a device (computer, access point, NAS) to a switch port.

A crossover cable uses T568A on one end and T568B on the other, swapping the transmit and receive pairs so two like devices (two computers, two switches) can communicate directly without a switch between them. Crossover cables were once required for direct PC-to-PC connections and switch-to-switch uplinks. Modern equipment universally implements Auto-MDI-X (Automatic Medium-Dependent Interface Crossover), which detects the cable type and automatically swaps the pairs in hardware as needed. This makes crossover cables functionally obsolete — a straight-through cable works in every scenario on current hardware.

Cable categories compatible with RJ45

PoE over RJ45: which pins carry power

Power over Ethernet delivers DC power alongside data over the same RJ45 connector and cable — no separate power cable needed for access points, IP cameras, VoIP phones, and switches. The IEEE 802.3 PoE standards define how power is applied:

• 802.3af (PoE, 15.4W): power applied as a common-mode voltage on the data pairs (pins 1/2 and 3/6) or the spare pairs (pins 4/5 and 7/8), depending on the PSE implementation.
• 802.3at (PoE+, 30W): same pin usage, higher power budget per port.
• 802.3bt (PoE++, 60W/100W): uses all four pairs simultaneously to deliver up to 90W to the powered device (Type 4). Required for high-power devices like PTZ cameras and multi-radio Wi-Fi 6E access points.

The connector itself is identical whether PoE is in use — the RJ45 jack on the powered device simply includes circuitry to extract the DC voltage from the pairs. Cable quality matters for high-wattage PoE: Cat5e handles up to PoE+; Cat6a is recommended for 802.3bt to manage heat from higher currents over longer runs.

Maximum segment length: 100 m

IEEE 802.3 specifies 100 metres as the maximum copper Ethernet segment length for 10BASE-T through 10GBASE-T. This 100 m budget is shared between the permanent link (typically 90 m of in-wall horizontal cable) and patch cables at each end (up to 10 m combined — usually a 3 m patch at the wall outlet and a 5 m patch at the switch). Beyond 100 m, signal attenuation and crosstalk cause bit errors and link instability. To extend beyond 100 m, place a switch at the midpoint (each switch port resets the 100 m clock) or use fibre with SFP transceivers, which have no meaningful distance limitation for building-scale runs.

Terminating an RJ45: tools and process

Field-terminating an RJ45 connector requires three steps and a small set of tools. First, strip approximately 25 mm of the cable jacket using a cable stripper, being careful not to nick the conductor insulation. Untwist the pairs only as far as necessary (excessive untwisting increases crosstalk) and arrange the eight conductors in the correct T568A or T568B colour order. Trim them to equal length (about 12–14 mm from the jacket edge) with cable cutters. Second, insert the conductors into the RJ45 plug — each conductor slides into its own channel and must reach the front of the connector to make contact with the pin. Third, place the plug in a crimp tool and squeeze firmly; the crimp tool drives the eight metal pins through the conductor insulation, making electrical contact, and sets the strain-relief boot onto the jacket. A cable tester with a remote loopback adapter verifies continuity and correct pin mapping before the cable is put into service. Punch-down connectors (keystone jacks for wall plates) use an IDC (Insulation Displacement Contact) tool instead of a crimp tool but follow the same colour-code standards.

RJ45 vs SFP for longer runs

RJ45 copper Ethernet is limited to 100 metres per segment. For runs beyond 100 metres — between buildings, across a campus, or up a long riser — use SFP fibre modules. SFP+ and QSFP ports accept fibre transceivers that reach hundreds of metres to tens of kilometres with no distance-related signal degradation. Many enterprise switches offer both RJ45 and SFP ports. For runs under 100 metres, RJ45 over Cat5e or Cat6 remains simpler, cheaper, and supports PoE where fibre cannot.

Frequently Asked Questions

What is the difference between T568A and T568B wiring?

The two standards differ only in which colour pairs occupy pins 1/2 and 3/6. T568B is more common in North America; T568A is the international default. Electrical performance is identical. The critical rule is consistency: both ends of a straight-through cable must use the same standard. Auto-MDI-X on modern equipment makes crossover cables (T568A one end, T568B the other) obsolete.

Can RJ45 carry PoE?

Yes. The same 8P8C connector and standard Cat5e/Cat6 cable carries both data and DC power. 802.3af/at uses the data pairs or spare pairs; 802.3bt uses all four pairs for up to 90W. The connector is identical regardless — power capability is determined by the switch port and the powered device's circuitry.

What is the maximum cable length for RJ45 Ethernet?

100 metres (328 feet) total per the IEEE 802.3 standard, including patch cables at both ends. Beyond 100 m, use a switch as a midpoint repeater, or switch to fibre (SFP) for longer building or campus runs with no distance penalty.