Using the Telnet Utility

Part of the TCP/IP protocol suite, Telnet is a virtual terminal protocol utility that allows you to make connections to remote devices, gather information, and run programs. Telnet was originally developed to open terminal sessions from remote Unix workstations to Unix servers. Although it’s still used for that purpose, we now use it as a troubleshooting tool as well. Figure 17.14 shows the basic Telnet interface as it’s being used to start a terminal session on a remote Unix host.

Figure 17.14 The Telnet utility

In today’s Windows environments, Telnet is a basic command-line tool for testing TCP connections. You can telnet to any TCP port to see if it’s responding—something that’s especially useful when checking Simple Mail Transfer Protocol (SMTP) and HTTP (web) ports.

As you learned back in Chapter 6, “Introduction to the Internet Protocol,” each upper-layer service in a TCP stack has a number for...