Telnet
4.2.1
This page will introduce Telnet.
Telnet is a virtual terminal protocol that is part of the TCP/IP protocol suite. It allows connections to be made to remote hosts. Telnet provides a network terminal or remote login capability. Telnet is an IOS EXEC command used to verify the application layer software between source and destination. This is the most complete test mechanism available.
Telnet functions at the application layer of the OSI model. Telnet depends on TCP to guarantee the correct and orderly delivery of data between the client and server.
A router can have simultaneous incoming Telnet sessions. The numbers zero through four are used to specify five vty or Telnet lines.
The verification of application layer connectivity is a by-product of Telnet. Telnet is mainly used to establish remote connections to network devices. Telnet is a simple and universal application program.
The next page will explain how a Telnet connection is initiated and verified.
Establishing and verifying a Telnet connection
4.2.2
This page will show students how to establish and test a Telnet connection.
The Telnet IOS EXEC command allows a user to Telnet from one Cisco device to another. In the Cisco implementation of TCP/IP, it is not necessary to enter the connect or telnet commands to establish a Telnet connection. The hostname or the IP address of the remote router may be entered. To end a Telnet session, use the EXEC commands exit or logout.
To initiate a Telnet session any of the following alternatives can be used:
Denver>connect paris
Denver>paris
Denver>131.108.100.152
Denver>telnet paris
A hostname table or access to DNS for Telnet must be present for a name to work. Otherwise, the IP address of the remote router must be entered.
Telnet can be used to determine if a remote router can be accessed. As shown in Figure , if Telnet is used successfully to connect the York router to the Paris router, then a basic test of the network connection is successful. This operation can be performed at either the user or privileged EXEC levels.
If remote access can be obtained through another router, then at least one TCP/IP application can reach the remote router. A successful Telnet connection indicates that the upper-layer application functions properly.
If Telnet to one router is successful, failure to another router is likely caused by addressing, naming, or access permission problems. The problem may exist on the original router or on the router that failed as a Telnet target. The next step is to use the ping command, which is covered later in this lesson. The ping command can be used to test end-to-end connections at the network layer.
Once the Telnet is completed, log off the host. The Telnet connection will terminate after ten minutes of inactivity by default or when the exit command is entered at the EXEC prompt.
Students can use the Lab Activity on this page to establish and verify a Telnet connection.
The next page will explain how to disconnect or suspend a Telnet connection.
