What is a RJ-45 Port?

Kurt Inman

An Registered Jack 45 (RJ-45) port is a data port often found on computers, network routers, switches and hubs. It is commonly used for an Ethernet or serial connection with an 8 position 8 conductor (8P8C) jack. In the past, some modems and telephones included a port that used a true RJ-45 jack. Although the 8P8C jack is not identical to the RJ-45, the data port commonly retains the name.

RJ45 ports on the back side of a router.
RJ45 ports on the back side of a router.

The most common RJ-45 port uses four of the 8P8C wires for Ethernet communications. Pins 1 and 2 are the transmit pair, while pins 3 and 6 are the receive pair. The port generally connects devices using a Category 5 (CAT5) or Category 6 (CAT6) twisted-pair cable. This port and cable combination typically supports communication speeds of up to 1 gigabit per second (Gbps). Unlike a true RJ-45 jack, the 8P8C jack includes all eight conductors and does not have an extra "key" tab.

Cat 5 cable with RJ45 plug.
Cat 5 cable with RJ45 plug.

An RJ-45 port for serial communications may use all eight wires of the 8P8C modular connector. Some Integrated Services Digital Network (ISDN) connections often use up to six of those wires. Two conductors may carry power while the other two are Tip/Ring signals. Another type of ISDN device may use four wires for transmit/receive pairs instead of two for Tip/Ring.

The original port design for use with a modem — now obsolete — followed the RJ-45 specification. The true jack has eight positions, but only two conductors that carry Tip/Ring signals. Pins 7 and 8 short out a resistor on the device, indicating the presence of a cable. RJ-45 is one of several registered jacks used for telephone connections. Registered Jack 11 (RJ-11) is almost always used for household phone wiring.

A modern gigabit Ethernet device is better served with a modified RJ-45 port. Augmented RJ-45 (ARJ45) and GigaGate 45 (GG45) are newer standards designed for high-speed communications. Both support 10 Gbps and faster connections using CAT6 and Category 7 (CAT7) cables. ARJ45 connectors come in two varieties: one for 8-pin CAT6 cables, the other for 12-pin CAT7 cables. In contrast, one GG45 connector can accommodate either type of cable.

Siemon Labs' TERA is a different type of high-speed connector. It does not use the RJ-45 form factor, but can be plugged into the same port with a special patch cord. Often used to carry broadcast video, it is also designed for speeds of 10 Gbps and faster.

A network cable tester can be used to determine if an issue lies with the cable or the RJ-45 port.
A network cable tester can be used to determine if an issue lies with the cable or the RJ-45 port.

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Discussion Comments


I've been using RJ45 for years and will continue to in our installations, with people now using 12v for lighting and various appliances Rj45 has been revitalized.

I'm using POE to send 48 volts through the extra wires, and then using switch supplies at the end to adjust to 12volt for lighting etc.

So the RJ45 is far from dead, with signal and power in a single wire, that is very hard to beat.

It' s useful in home automation and surveillance, and is very often used in modern energy low powered houses, instead of the standard 220-250v we use here.


@summertime, I agree with your thoughts on a new connection type for computer networking, I just hope that it is as easy as the older style coaxial connections to create ends for. Those were the days.

If it is going to be successful in the marketplace, it will have to be easy to use for modern day information technology professionals.


While I agree with the previous comment concerning the very easy of use that RJ-45 connectors inherently now have because of it's time on the market, I do believe that we need to move on in our efforts to keep up with increasing network traffic.

Eventually we will need to push our technology sectors and companies toward using a faster connection type. While fiber optic networking has the amazing ability to transfer insane amounts of data in a very short amount of time, there is problems using this technology on a consumer level. Not many at home users will have any clue how to properly polish the glass connections that require high levels of training to complete. It's just not practical. What we need is a solid way of creating home networking cables that exceed the current gigabit standards. Better yet, they should be wireless.


I remember the first time I had ever attempted to change and crimp an RJ-45 connection. As the author states, there are eight individual wires in four twisted-pairs inside of the category 5, 5e or 6 wiring that is used in conjunction with RJ-45 connectors. This great amount of small wiring being placed into exact slots in the tip of an RJ-45 connector makes the task extremely difficult.

Luckily, the job I was doing to replace a gigabit switch 8 port box came with a testing device that ensured my connections were solid. It still took too long and over three failed attempts to get both ends of the new uplink cable connected at full strength and proper pin output.


It is amazing to me that that the RJ-45 network port has been able to remain in existence this entire time. Not many computer systems or types of cable connectors and protocols last more then a decade and the RJ-45 tip is one that certainly has proved it's worth. Used in essentially every type of computer network setup, the rj45 cable is ubiquitous in offices, houses and military complexes across the globe.

I think it is important for us to continue to keep standards such as this to ensure that compatibility across systems can remain easy to use. It drastically reduces costs when we are able to use the same test and connection equipment for many years instead of changing standards too often for service companies to stay profitable.

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