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Computer Networks · Class 12 · Network Devices
networksdevices⏱️ 4 min read

RJ45 Connector

Follow the blue cable out of the back of a computer in your lab. At each end there is a small clear plastic plug that clicks into place. That plug has a name, and it is on every exam paper.

1What RJ45 is

RJ45
RJ45 (Registered Jack 45) is an eight-pin connector used on the ends of a network cable (Ethernet cable / LAN wire). It plugs the cable into the RJ45 port — the opening — on a NIC (Ethernet card), a hub, a switch or a router.

The word jack means a plug-and-socket connector. Strictly, the plug on the cable and the port it goes into are both called RJ45. In the lab below, the port is the one on a computer’s NIC — the card that bears the computer’s MAC address.

An RJ45 plug and a NIC's port

Plug it into the NIC's RJ45 port. Zoom in, number the pins, and compare it with a telephone plug. Drag to turn.

The whole network card, to scale: the RJ45 port is a small part of it, at the end by the metal bracket — the only part you see from the back of a computer.

2Its parts

🟨 Eight contacts

Eight gold contacts, all the same size, each in its own slot across the tip — one for each of the eight wires inside the cable.

🧊 Clear plastic body

You can see the coloured wires through it, which makes a badly made cable easy to spot.

🔒 The latch

A springy tab on the opposite side from the contacts. It locks the plug into the port; press it to pull the plug out.

The eight wires are twisted together in four pairs inside the cable. Each wire must reach the right pin, so cables are made in a fixed colour order. Press Number the pins in the lab to see the most common order. You will not be asked to learn the colours.

3Not the telephone plug

A landline telephone uses a plug that looks very similar, called RJ11. It is easy to mix them up.

🔌 RJ45
  • Used on network cables (Ethernet cable / LAN wire)
  • Wider
  • 8 contacts, 8 wires
📞 RJ11
  • Used on telephone cables
  • Narrower
  • Fewer contacts — usually 2 or 4
RJ45 is a connector, not a device that thinks
An RJ45 connector does nothing to the data. It only joins a cable to a port. It is in the list of network devices because no wired network works without it.

4Check yourself

Quick Check

What does RJ45 stand for?

Quick Check

How many pins does an RJ45 connector have?

Quick Check

Which connector is used on an Ethernet cable (LAN wire) to plug into the port of a network card?

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🧩 Ethernet Card (NIC)
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Wi-Fi Card 📶