What Is RJ45?

RJ45 is the everyday name for the eight-position, eight-contact (8P8C) modular connector on the end of an Ethernet cable and on every wired network port. The clear plastic plug has a springy latch that clicks into the jack, and eight gold-plated contacts line up with the eight wires of a twisted-pair cable.

RJ stands for registered jack, a series of telephone wiring codes from the US Bell System. The original RJ45S was a keyed modem jack that is now obsolete. Ethernet adopted an unkeyed connector of the same size, and the RJ45 name stuck. Today the connector is defined by IEC 60603-7, and the wiring by the TIA-568 cabling standard.

An Ethernet port, or LAN port, is the RJ45 socket on a computer, switch, camera or router. Behind it sits an Ethernet controller and magnetics that set the speed, from 10 Mb/s up to 10 Gb/s, rather than the connector itself.

Yes, in almost every case. A LAN port is an Ethernet port named for what it connects to: the local area network. On a computer, switch, camera or industrial PC the two names describe the same RJ45 socket, driven by the same Ethernet controller, and a port labelled LAN1 or LAN2 is simply the first or second Ethernet port.

The distinction matters on routers and gateways. There, the LAN ports connect devices on the local network, while a separate WAN or internet port, often the same RJ45 socket in a different colour, connects to the outside network or the modem. Plugging the uplink into a LAN port is one of the most common set-up mistakes.

Industrial computers often carry several LAN ports, each with its own controller, so cameras, machines and the plant network can be kept on separate networks. Some add PoE on selected ports or move the ports onto M12 connectors. Whether the Ethernet ports sit on the motherboard or on an add-in card, our explainer on the network interface card covers how they are built.

Two wiring schemes are in use. Both connect the same four pairs and perform identically. The only rule is to use the same scheme at both ends of a cable, or a crossover is created. T568B is the more common choice for patch leads, while some government and residential specifications call for T568A.

Fast Ethernet uses only two pairs. Gigabit and faster Ethernet use all four in both directions, so every pin must be terminated. Modern ports detect a crossed cable automatically (auto-MDIX), so crossover cables are rarely needed now.

Power over Ethernet uses the same eight pins. IEEE 802.3 defines two ways to add power: Alternative A puts it on the data pairs at pins 1 and 2 and pins 3 and 6, and Alternative B on the other pairs at pins 4 and 5 and pins 7 and 8. Powered devices must accept either. Four-pair PoE under IEEE 802.3bt uses all four pairs at once, so a cable for a PoE camera needs every pin terminated, even when the camera only runs at 100 Mb/s.

The RJ45 connector does not limit speed on its own. The cable category and the port’s Ethernet controller do. Under IEEE 802.3, a twisted-pair Ethernet channel can run up to 100 m, typically 90 m of fixed cable plus patch leads, at every speed up to 1 Gb/s on Cat5e. 10GBASE-T needs Cat6A for the full 100 m, or reaches around 55 m on Cat6.

For longer runs or higher speeds between switches, a fibre or direct-attach link is usually the better fit. Our comparison of SFP+ and RJ45 for 10GbE links covers when each one suits.

The RJ45 socket is the most common Ethernet port, but not the only one. Ethernet ports are usually described by their connector and their speed, and industrial computers and switches often mix several types.

Standard RJ45 jacks are built for offices. Their plastic latch can break, and the open jack lets in dust and moisture. On machines, vehicles and outdoor cabinets, industrial designs address that in several ways.

Field termination is simple with the right tools, but most intermittent network faults trace back to a poorly made plug. The usual sequence is below.

• Strip and untwist. Remove around 25 mm of jacket, then untwist each pair only as far as needed, keeping the twists within about 13 mm of the contacts to preserve the cable’s performance.
• Order the wires. Lay the eight conductors flat in T568A or T568B order, trim them square and push them fully into the plug so each one reaches the end of its channel.
• Crimp. Use a ratchet crimp tool sized for the plug, so the contacts pierce every conductor and the strain relief grips the jacket.
• Test. A cable tester confirms the pin map, and a certifier checks the link against its category for permanent installations.

Match the plug to the cable: solid-core installation cable and stranded patch cable need different contacts, and shielded cable needs a shielded plug. Where cameras or access points draw power over the same cable, good terminations matter more, because a high-resistance contact heats up under Power over Ethernet loads.

We fit the network cards, switch ports and connector types each installation needs as standard, including M12 and PoE where the site calls for them, and test every link before the system ships. Send us your device list and cable runs, and we will specify the ports to match.