Another way to learn
about a remote router is to connect to it. Telnet, a virtual terminal
protocol that is part of the TCP/IP protocol suite, allows connections to
be made to hosts. You can set a connection between a router and a connected
device. Telnet allows you to verify the application-layer software between
source and destination stations. This is the most complete test mechanism
available. A router can have up to five simultaneous incoming Telnet
sessions.
Let's begin testing by
initially focusing on upper-layer applications. As shown in Figure , the telnet command provides a virtual terminal so administrators can use
Telnet operations to connect with other routers running TCP/IP.
With Cisco's
implementation of TCP/IP, you do not need to enter the command connect or telnet to establish a Telnet
connection. If you prefer, you can just enter the learned host name. To end
a Telnet session, use the EXEC commands exit or logout.
The following list
shows alternative commands for the operations listed in the figure:
- Initiate a session from
Denver:
Denver> connect paris
Denver> paris
Denver> 131.108.100.152
- Resume a session (enter
session number or name):
Denver>1
Paris>
- End a session:
Paris> exit
As you have already
learned, the Telnet application provides a virtual terminal so that you can
connect to other hosts that are running TCP/IP. You can use Telnet to
perform a test to determine whether or not you can access a remote router.
As is shown in Figure , if you
can successfully use Telnet to connect the York router to the Paris router,
then you have performed a basic test of the network connection.
If you can remotely
access another router through Telnet, then you know that at least one
TCP/IP application can reach the remote router. A successful Telnet
connection indicates that the upper-layer application (and the services of
lower layers, as well) function properly.
If we can Telnet to one
router but not to another router, it is likely that the Telnet failure is
caused by specific addressing, naming, or access permission problems. These
problems can exist on your router or on the router that failed as a Telnet
target. The next step is to try ping, which is covered in this
section. This command lets you test end-to-end at the network layer.
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Lab Activity
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In
this lab, you will work with the telnet (remote terminal) utility
to access routers remotely. You will telnet from your
"local" router into another "remote" router in
order to simulate being at the console on the remote router.
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