RJ45 is the common name for the eight-position, eight-contact modular connector that terminates twisted-pair Ethernet cables and appears on the copper ports of switches, routers, computers, access points and patch panels. In networking and network hardware, it is the standard physical interface for wired Ethernet at speeds from 10 Mbit/s up to 10 Gbit/s over copper.
Strictly speaking, the connector is an 8P8C modular plug and jack; the name RJ45 originally referred to a telephone wiring arrangement that used the same shell. The networking industry adopted the older name, and today RJ45 means the Ethernet connector in almost every technical context, including datasheets.
How the RJ45 connector is built
An RJ45 connection consists of a plug on the cable and a jack in the device or wall outlet. The plug holds eight gold-plated contacts that pierce or press onto the eight conductors of the cable, and a flexible latch locks it into the jack. The jack contains eight spring contacts that press against the plug’s contacts when it is inserted.
The eight conductors form four twisted pairs, and the twisting is what lets unshielded copper carry high frequencies with limited interference. Inside the plug, the pairs must be untwisted only for a few millimeters; untwisting more than about 13 mm degrades crosstalk performance and can make a link fail certification.
| Type | Description | Typical use |
|---|---|---|
| Unshielded (UTP) plug | Plastic body without metal shield | Office and home cabling, CAT5e and CAT6 |
| Shielded (STP or FTP) plug | Metal shell bonded to the cable shield | CAT6A and higher, electrically noisy areas |
| Pass-through plug | Conductors pass through the front for easy trimming | Field termination of patch cables |
| Keystone jack | Snap-in jack with punch-down terminals | Wall outlets and modular patch panels |
| Field-terminable plug | Tool-less plug for solid installation cable | Direct connection of cameras and access points |
RJ45 pin assignments and wiring standards
Two wiring schemes, T568A and T568B, define which colored conductor goes to which pin. They differ only in swapping the orange and green pairs, and both perform identically, but one scheme should be used consistently across an installation to avoid confusion.
| Pin | Conductor color | Pair | Use in 100BASE-TX |
|---|---|---|---|
| 1 | White-orange | Pair 2 | Transmit + |
| 2 | Orange | Pair 2 | Transmit – |
| 3 | White-green | Pair 3 | Receive + |
| 4 | Blue | Pair 1 | Unused |
| 5 | White-blue | Pair 1 | Unused |
| 6 | Green | Pair 3 | Receive – |
| 7 | White-brown | Pair 4 | Unused |
| 8 | Brown | Pair 4 | Unused |
Fast Ethernet uses only two pairs, but Gigabit Ethernet and faster use all four pairs in both directions at once. A cable with a damaged blue or brown pair may therefore work at 100 Mbit/s yet fail at 1 Gbit/s, and devices often fall back silently to the lower speed.
RJ45 compared with other network connectors
RJ45 is the dominant copper connector at the access layer, while fiber and direct-attach connections take over at higher speeds and longer distances. The table places it among its neighbors.
| Interface | Medium | Typical speeds | Typical reach |
|---|---|---|---|
| RJ45 | Twisted-pair copper | 1, 2.5, 5 and 10 Gbit/s | Up to 100 m |
| SFP | Fiber or copper module | 1 Gbit/s | Up to many kilometers with fiber |
| SFP+ | Fiber, DAC or copper module | 10 Gbit/s | Meters with DAC, kilometers with fiber |
| LC fiber connector | Optical fiber | 1 to 400 Gbit/s | Hundreds of meters to kilometers |
| RJ11 | Telephone copper | Not used for Ethernet | Telephone lines and DSL |
Copper 10 GbE over RJ45 draws noticeably more power per port and produces more heat than SFP+ with fiber or direct-attach cables, which is why data centers prefer SFP+ and RJ45 dominates in offices, homes and access-layer switches.
Speed, cable category and power
The RJ45 connector itself does not determine speed: the cable category, the termination quality and the link length do. A connector and jack must be rated for the same category as the cable, or the whole link is limited to the weakest component.
- CAT5e supports 1 Gbit/s at 100 m and usually 2.5 Gbit/s.
- CAT6 supports 1 Gbit/s at 100 m and 10 Gbit/s up to about 55 m.
- CAT6A supports 10 Gbit/s over the full 100 m channel.
- PoE delivers power through the same RJ45 port, up to about 90 W at the source with IEEE 802.3bt Type 4.
High-power PoE heats cable bundles and can cause arcing damage to contacts if a plug is pulled under load, so connectors rated for the full PoE power class are recommended for cameras, access points and other powered devices.
Common RJ45 faults and their causes
Most RJ45 problems come from termination errors or mechanical wear, not from the electronics. A simple cable tester finds wiring faults, while certification testers measure crosstalk and return loss against category limits.
| Symptom | Likely cause | Fix |
|---|---|---|
| No link at all | Open conductor or wrong pin order | Re-terminate and test continuity |
| Link at 100 Mbit/s instead of 1 Gbit/s | One of the four pairs is broken | Re-terminate or replace the cable |
| Intermittent drops when the cable moves | Broken latch or worn jack contacts | Replace the plug or the port |
| Errors on long runs | Split pairs or excessive untwisting | Follow T568A or T568B exactly |
A non-technical view
RJ45 is the small clear plastic plug at the end of a network cable. It clicks into the back of a computer, a router or a wall socket. It looks like a slightly wider telephone plug and holds eight tiny metal contacts in a row.
Each contact connects to one thin colored wire inside the cable. The wires must sit in the correct order, and the plug must be pressed on firmly. Then the cable can carry fast, stable internet and even power for a device such as a security camera. A wrongly ordered or damaged plug is a common reason for a slow or dead wired connection.
A concrete example
An electrician installs CAT6A cabling in a newly renovated dental clinic, terminating each run on a shielded keystone jack in a wall outlet and on a shielded patch panel in the server cabinet. Every jack is wired to T568B, and every link is certified for 10 Gbit/s over its full length before the clinic moves in.
Months later, the X-ray workstation in one treatment room negotiates only 100 Mbit/s, and image transfers take minutes. A tester shows that the brown pair is open in a short patch cable whose plug was crushed under a cabinet wheel. Replacing the patch cable restores the 1 Gbit/s link, and image transfers return to a few seconds without any change to the installed cabling.