Using the Telnet Utility

Using the Telnet Utility

Telnet is an acronym formed from Terminal EmuLation for NETworks. It was originally developed to open terminal sessions from remote UNIX workstations to UNIX servers. Although still
used for that purpose, it has evolved into a troubleshooting tool. Figure 4.12 shows the basic Telnet interface as it is being used to start a terminal session on a remote UNIX host.

FIGURE 4 . 1 2 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 is responding, which is especially useful when checking SMTP and HTTP (Web) ports. If you’ll remember from Chapter 3, each upperlayer service in a TCP stack has a number for its address. Each network service that uses a particular address will respond to a TCP request on this port (if the defaults are used). Table 4.6 lists the most commonly referenced port numbers and their associated services.

TABLE 4 . 6 TCP Port Numbers and Services

NOTE:

This list is by no means comprehensive. For a complete list, go to www.iana.org/assignments/port-numbers.

To find out if a TCP service is responding, follow these steps:

1. Choose Start Run.
2. Type Telnet and click OK to open the Telnet utility.

3. At the prompt, enter a question mark (?) to display the valid commands for this interface.

4. At the prompt, type open and the IP address or DNS host name of the device running the TCP service to which you want to connect, followed by the TCP port number (one from

Table 4.6, for instance). 

For example, to find out if the Sybex web server is responding to TCP port 80 (its default port), enter the IP address or DNS host name of the server (www.sybex.com, in this case)

followed by the TCP port number (80, in this case).

NOTE:

If you successfully connect to the web server, you won’t be notified that this is the case. If the web server doesn’t respond, you’ll receive a Connect Failed message.

5.If the procedure was successful, you can simply close the Telnet window or key in Ctrl+] to return to the Telnet prompt.