Chapter 21 · Network Utilities 211 T EL N ET C OMMAND Telnet (TELetype NETwork), uses TCP port 23 as a way to establish virtual client-server (see "Client-Server," on page 25) "terminal" sessions over a network, enabling a user to type and receive characters just as if he or she was sitting at the remote computer. The telnet protocol can operate in three modes: half-duplex (rarely used), one character at a time, or one line at a time. To allow interoperability over a wide variety of systems, terminal types or "emulations" are used, since in the old days, each terminal manufacturer would map out their own way of dealing with characters and keys. Telnet is both a protocol and a program, so people often get confused between the two, although telnet programs will typically default to the telnet proto- col port 23 and use the telnet protocol. However, you can also communicate using ports other than port 23, and this makes some telnet programs useful as a general, low-level diagnostic tool. So, someone saying "I telnet'd to that piece of gear over the network" might actually be saying that they used the telnet program to send and receive raw ASCII characters--see "American Standard Code for Information Interchange (ASCII)," on page 140--to and from a remote device, with the characters packaged up into TCP/IP messages by the telnet program and using some port other than 23. Windows and other OSs come with a version of telnet (type telnet from the command line to start it). However, (in Windows anyway) this telnet program is limited, so most people pre- fer to use another program, such as Tera Term (see "Communicating With and Testing Serial Connections," on page 167), which, in addition to point-to-point serial communications, can be used for telnet and ASCII communications via IP.