Computers in a small area such as a home or office are connected on a local area network, or LAN. A computer technician creates a LAN by running Ethernet cable throughout the building and plugging one end of the cable into a computer. The other end of the cable connects to a router or modem, which then connects to the Internet by other means. The cables are secured to a special plug on the computer called the Ethernet port, though this is colloquially referred to as an LAN jack. LAN jacks are usually located on the back of the computer as part of the network interface controller, or NIC.
The standard LAN jack is a configuration known as the RJ45. "RJ" stands for "registered jack," a standardised type of physical interface. RJ45 is technically an incorrect term, but the jack has inherited the nickname over time from similar-looking but obsolete interfaces used for telephones. Today, telephones use the much smaller RJ11 interface. The proper name for an RJ45 is an 8P8C connector.
In locations with more than one computer, a router is necessary to send and receive an Internet signal to and from each computer. These routers contain a series of LAN jacks. The router also has a final port labelled "out," "modem" or something similar. This jack is used to connect the router to the modem. Many modern routers now contain wireless signals in addition to LAN jacks, allowing users the choice of which connection they prefer.
Modems are used to connect computers to the Internet. Older, phone-based modems have been replaced by cable modems and fibre-optics modems, known collectively as broadband Internet. Modems contain one LAN jack, which connects to either a router or one computer using an Ethernet cable. For home users, modems are often provided by the user's Internet service provider, or ISP.
Some modern modems have been combined with routers in order to simply the configuration of the network. In this case, the user need only connect all of the computers in the building to one single piece of hardware. These modems contain a row of LAN jacks, a special port for connection to the Internet and a wireless receiver.