What is RJ45?

Before the name RJ45 was used to refer to computer networking connectors, RJ45 was originally a telephone-only standard. It is one of the many registered jacks, like RJ-11, another telephone standard. As a registered jack, telephone RJ45 specifies the physical male and female connectors as well as the pin assignments of the wires in a telephone cable. The original RJ45 uses a special keyed 8P2C modular connector, with Pins 5 and 4 wired for tip and ring of a single telephone line and Pins 7 and 8 connected to a programming resistor. It is meant to be used with a high speed modem, and is obsolete today.

Telephone installers who wired telephone RJ45 jacks were familiar with the pin assignment which was part of the RJ45 standard. However, near-identical physical connectors for computer networking became ubiquitous, and informally inherited the name RJ45 due to the overwhelming similarity. While telephone RJ45 uses a "keyed" variety of the 8P body, meaning it may have an extra tab that a computer RJ45 connector is unable to mate with, the visual difference from an Ethernet 8P is subtle.

The only other difference is the presence of extra conductors in the cable, which cannot be seen without very close inspection. True telephone RJ45 connectors are a special variant of 8P2C, meaning only the middle 2 positions have conductors in them, while pins 7 and 8 are shorting a programming resistor. Computer RJ45 is 8P8C, with all eight conductors present.

Understandably, because telephone RJ45 8P connectors never saw wide usage and computer 8P connectors are quite well known today, RJ45 is used almost exclusively to refer to Ethernet-type computer connectors. Electronics catalogs not specialized to the telephone industry advertise 8P8C modular connectors as "RJ45". Virtually all electronic equipment that uses an 8P8C connector (or possibly any 8P connector at all) will document it as an "RJ45" connector. In common usage, RJ45 also refers to the pin assignments for the attached cable, which are actually defined in the wiring standard TIA/EIA-568-B.

A locking boot for RJ45 Modular Plugs were developed to prevent inadvertent disconnects in mission critical facilities.