Roughly one billion years ago there was no such thing as the Internet or even networks... Well, maybe it was only about 40 years ago, but as far as nerds like me are concerned, a world before the Internet was filled with Brontosauruses and palm fronds. The only computers were huge monsters called mainframes and to access them required a dumb terminal.
Operating systems didn't have windows and pretty icons. The interface to the mainframe was a command line, but it worked just fine for the time. The cavemen who first lifted their heads up from the computer ooze known as mainframes said to themselves, "Wouldn't it be great if we could access each other's computers from the comfort of our own caves?" That was what started the entire concept of a network. Back then the idea of sharing folders or printers or Web pages wasn't even yet considered. The entire motivation for networking was so people could sit at their dumb terminals and, instead of accessing only their local mainframes, access totally different mainframes. The protocol to do this was called the Telnet Protocol or simply Telnet.
Even though PCs have replaced mainframes for the most part, Telnet still exists as the way to connect remotely to another computer via the command line. Telnet runs on TCP port 23, enabling you to connect to a Telnet server and run commands on that server as if you were sitting right in front of it.
This way, you can remotely administer a server and communicate with other servers on your network. As you can imagine, this is sort of risky. If you can remotely control a computer, what is to stop others from doing the same? Thankfully, Telnet does not just allow anyone to log on and wreak havoc with your network. You must enter a user name and password to access a Telnet server. Unfortunately, Telnet does not have any form of encryption. If someone intercepted the conversation between a Telnet client and Telnet server, he or she would see all of the commands you type as well as the results from the Telnet server. As a result, Telnet is rarely used on the Internet and has been replaced with Secure Shell (SSH), a terminal emulation program that looks exactly like Telnet but encrypts the data.
NOTE Telnet only enables command-line remote access, not from a GUI. If you want to access another computer's desktop remotely, you need another type of program.
Even though Telnet is less common than SSH, Telnet is a popular second option to connect to almost anything on a trusted TCP/IP network. Most routers have Telnet access capability (although many router admins turn it off for security). Almost every operating system has a built-in Telnet client and most operating systems-though not all Windows operating systems-come with built-in Telnet servers. Almost every type of server application has some way for you to access it with Telnet. It was once quite common, for example, to administer Apache-based Web servers through Telnet.
NOTE By default, Windows Server 2008 does not have Telnet Server or Telnet Client installed, though you can install both through Server Manager | Features | Add Features. Windows Vista, likewise, does not have Telnet Client installed. It is available for quick installation in Programs and Features.
In this tutorial:
- TCP/IP Applications
- Transport Layer Protocols
- The Power of Port Numbers
- Registered Ports
- Connection Status
- Rules for Determining Good vs. Bad Communications
- Common TCP/IP Applications
- Publishing Web Pages
- Web Servers and Web Clients
- Secure Sockets Layer and HTTPS
- Telnet Servers and Clients
- Configuring a Telnet Client
- Rlogin, RSH, and RCP
- SSH and the Death of Telnet
- SMTP, POP3, and IMAP4
- Alternatives to SMTP, POP3, and IMAP4
- E-mail Servers
- Passive vs. Active FTP