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The RJ45 is similar to the RJ11, except its connector has eight pins, and it serves as the interface for ethernet networks instead of telephone networks. The clear plastic jacks of the RJ45 usually have a Cat 5E cable running between each jack. Creating a connection between a network device like a switch or router and a network card. The other place you will see this type of connection is on a patch panel. These are usually found in the server closet. In a normal install, this will be labeled to correspond with a jack out in the network for easy identification.

RJ45 (Wikipedia)
Left to right, modular connectors:
  • eight-contact 8P8C plug used for RJ45S, RJ49, RJ61 and others.
  • six-contact 6P6C plug used for RJ25
  • four-contact 6P4C plug used for RJ14
  • four-contact 4P4C handset plug
  • 6P6C jack, may be used for RJ11, RJ14, and RJ25.

A registered jack (RJ) is a standardized telecommunication network interface for connecting voice and data equipment to a service provided by a local exchange carrier or long distance carrier. Registration interfaces were first defined in the Universal Service Ordering Code (USOC) system of the Bell System in the United States for complying with the registration program for customer-supplied telephone equipment mandated by the Federal Communications Commission (FCC) in the 1970s. They were subsequently codified in title 47 of the Code of Federal Regulations Part 68. Registered Jack connections began to see use after their invention in 1973 by Bell Labs. The specification includes physical construction, wiring, and signal semantics. Accordingly, registered jacks are primarily named by the letters RJ, followed by two digits that express the type. Additional letter suffixes indicate minor variations. For example, RJ11, RJ14, and RJ25 are the most commonly used interfaces for telephone connections for one-, two-, and three-line service, respectively. Although these standards are legal definitions in the United States, some interfaces are used worldwide.

The connectors used for registered jack installations are primarily the modular connector and the 50-pin miniature ribbon connector. For example, RJ11 uses a six-position two-conductor connector (6P2C), RJ14 uses a six-position four-conductor (6P4C) modular jack, while RJ21 uses a 25-pair (50-pin) miniature ribbon connector.

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