The telnet utility is a virtual terminal protocol that is part of the TCP/IP protocol suite. It allows verification of the application layer software between source and destination stations. This is the most complete test mechanism available. The telnet utility is normally used to connect remote devices, to gather information, and to run programs.

The Telnet application provides a virtual terminal for connection to routers running TCP/IP. For troubleshooting purposes, it is useful to verify that a connection can be made using Telnet. This proves that at least one TCP/IP application is able to connect end-to-end. A successful Telnet connection indicates that the upper-layer application and the services of lower layers are functioning properly.

If an administrator can Telnet to one router but not to another router, verify lower layer connectivity. If connectivity has been verified, it is likely that the Telnet failure is caused by specific addressing, naming, or access permission problems. These problems can exist on the administrator's router or on the router that failed as a Telnet target.

If the Telnet to a particular server fails from one host, try connecting from a router and several other devices. When trying to Telnet, if a login prompt is not achieved, check the following:

• Can a reverse DNS lookup on the client's address be found? Many Telnet servers will not allow connections from IP addresses that have no DNS entry. This is a common problem for DHCP-assigned addresses in which the administrator has not added DNS entries for the DHCP pools.
• It is possible that a Telnet application cannot negotiate the appropriate options and therefore will not connect. On a Cisco router, this negotiation process can be viewed using debug telnet.
• It is possible that Telnet is disabled or has been moved to a port other than 23 on the destination server.

Lab Activity Lab Exercise: Troubleshooting Using Ping and Telnet This lab is to gather information about routing updates and routing protocols.

Interactive Media Activity Point and Click: Telnet After completing this activity, the student will be able to understand Telnet.

Web Links telnet Command http://www.cisco.com/en/US/products/sw/ iosswrel/ps1835/ products_command_reference_ chapter09186a008008710e.html#998293