RJ45, which incidentally stands for registered jack 45, is the ubiquitous Ethernet style data port found on switches, routers and network cards. The RJ45 style connectors and ports are also commonly found on Ethernet, serial and ATM IMA cards and interfaces. However, the most common use by far is with Ethernet data connections from desktop PC network cards, Wi-Fi access points, data switches and routers in home and corporate networks.
RJ45 actually refers to the port (female) and the connector (male) though it often is used to refer to a cable type, which consists of two male connectors RJ45 connected by a length of cat 5 or cat 6 twisted pair cable. These are commonly referred to as Ethernet cables. The result is that confusion has arisen between the physical connectivity media, the RJ45 ports and cables found on network cards and switches, and the actual underlying Ethernet layer-2 network protocol. This is an important aspect that needs clarification, RJ45 is the specification for the physical connectors both male and female, it is not part of Ethernet, and it simply is the most common media for transmitting data in LAN environments. Ethernet after all can work over most media types such as infrared, Wi-Fi, microwave radio, fiber optics and satellite for short medium and long haul deployments. Similarly, RJ45 cables can be configured to support not just Ethernet but serial, ISDN T1 & E1 as well as ATM protocols.
The RJ45 specification and cable type became almost synonymous with Ethernet due to its ubiquitous presence in corporate LANs. However the inexpensive connectors and cat 5 standard cabling made constructing LANs within a business very economical. RJ45 cables using RJ45 connectors and cat 5 twisted pair cabling is easy to construct using the correct tools. In particular, RJ45 connectors and cabling standards are simplicity itself as RJ45 only uses four of the eight pins on the connectors. Pins 1 & 2 transmit data, whilst pins 3 & 6 are used to receive data when used in an Ethernet configured data port. In serial or ISDN environments, all the pins are wired in a straight through configuration and are used by the relevant protocols for control and transmission.