Another very useful TCP/IP utility is
telnet, which is standard on the UNIX operating system. Telnet is
a client-server terminal emulation program that allows you to connect
to another system. The telnet server simulates a terminal in order to
authorize a user or telnet client to connect to a remote system
and work in that environment. When you telnet to another host,
you are prompted for a use name and password. Once a session is
established from your telnet client, you can enter commands as
if you were entering them directly on the server console. You can
telnet to several different hosts and have multiple telnet sessions
open simultaneously.
All operating systems that support TCP/IP provide at least a telnet
client. Not all operating systems provide a telnet server or daemon,
however UNIX provides both. In addition to connecting to other hosts,
telnet can also be used to connect to and manage network devices such
as switches and routers. These devices are actually microcomputers
with a CPU and RAM but they do not have a keyboard or monitor. Telnet
allows you to use your computer's keyboard and monitor to remotely
access and administer these devices.
Telnet is another good network
troubleshooting tool. It is a non-graphical communications utility
that can be used to check the upper layers of the OSI model. It runs
at layer 7, the Application layer. If you are having trouble executing
another, perhaps graphical, client-server application, you can try to
telnet to the host or server to very that the TCP/IP protocol stack is
functioning correctly. Remember, not all network operating systems
support the telnet server function. For instance, you can telnet from
a Windows 9x or NT/2000 workstation to a UNIX server, but you cannot
telnet to the Windows workstation. The Windows operating system
includes a telnet client but not a telnet daemon or
server. As long as the host operating system is running a telnet
daemon, you can telnet to it.
You can telnet to the name of a host computer if you have a naming
service running such as Domain Name Service (DNS) or if you have
entered the host name and IP address in your /etc/inet/hosts
files. If not, you can telnet the IP address of the host. When
using telnet, you can:
- Open a session on a remote
machine
- Alternate between the remote
session and the local session
- Access machines that do not run
under the UNIX environment
Figure
shows an example of using telnet to connect to a remote system called
beach. Notice that the host name beach is converted to an IP address
using name to IP address resolution. You could also telnet directly to
the IP address of the host if you know it. Figure
is an example of using telnet to alternate between the remote session
and a local session.
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