|  | 4.4 | Basic Networking Testing
| 4.4.2 | Testing the application layer by using telnet
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 that 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.
| Lab Activity
| 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.