"RJ45" Naming Confusion
As mentioned above, the term "RJ45" may refer to two rather different things. To avoid ambiguity in this section, they will be referred to as true telephone RJ45 and computer "RJ45". Except where otherwise indicated, this article is about the computer "RJ45" alone.
Originally, there was only the true telephone RJ45. It is one of the many registered jacks, like RJ11, a standard from which it gets the "RJ" in its name. As a registered jack, true telephone RJ45 specifies a physical connector, and also the wiring of it. The true telephone 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 It is meant to be used with a high speed modem, and is obsolete today.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 installed true telephone RJ45 jacks in the past were familiar with the inner workings which made it RJ45, but their clients saw only a hole in the wall of a particular shape, and came to understand RJ45 as the name for a hole of that shape. When they found similar-looking connectors being used in entirely non-telephone applications, usually connecting computers, they called these "RJ45", too. This was therefore the so-called computer "RJ45".
Compounding the problem was the fact that the physical connectors indicated by true telephone RJ45 are not even compatible with computer "RJ45" connectors. 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 -- all eight conductors are always present. Furthermore, true telephone RJ45 involves a "keyed" variety of the 8P body, which means it may have an extra tab that a computer "RJ45" connector is unable to mate with.
Because true telephone RJ45 never saw wide usage and computer "RJ45" has become well known today, computer "RJ45" is almost always what a person is referring to when he says "RJ45". 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.
There have been conflicts when clients demand "RJ45" and an installer insists that they ask for something else or they will get something quite different from what they actually need.
Rounding out the confusion in "RJ45" naming is the fact that some people intend for the term to encompass not just the connector shape and size, but the wiring standard for it described by TIA/EIA-568-B as well. So one might find "Here is the pinout of an RJ45 jack."
