8.10 Implementing Installation of the Network
8.10.5 The uses of RJ-45 connectors may differ 
greatly depending on the cabling
Registered Jack (RJ) connectors were previously the standard for telephone connectors. More recently, RJs have been used to connect not only the telephones, but also 10BASE-T, 100BASE-TX, and Token Rings.

Telephone lines use the RJ-11 connector, which connects four-wire cables, the two inner connectors for one phone line and the two outer for another line. Modems are restricted to using RJ-11 because it is an analog connector. RJ-12, which is rarely used, is a six-wire version of the RJ-11 used for more complex telephone systems.

RJ-45 connectors have eight wires and are used for network technologies, which require four pairs of wires, such as Ethernet and Token Ring networks. RJ-45 connectors are specifically designed for digital signals. If an analog modem uses a RJ-45 connector, either the connection will not function or the analog modem will stop functioning and burn out. RJ-45 connectors are mainly used to connect 10BASE-T cabling (or the like) to a hub or bridge in order to connect to another media type, such as using a BNC connector for 10BASE2.