Core Issue
Users can access the Cisco router's Command-Line Interface (CLI) through Telnet by using vty sessions available on the router. Users can also Telnet to other devices from a Cisco router.
A router configured to restrict devices from which it is accessed through Telnet is known as the inbound Telnet access. A router configured to restrict devices from which a user can Telnet from the router is known as the outbound Telnet access.
Resolution
To specify devices to be denied or permitted, configure an Access Control List (ACL) in global configuration mode. A standard ACL is usually used for this purpose. Issue the access-list access-list-number {deny | permit} source [source-wildcard] command.
Perform these options:
For more information, refer to the Controlling Access to a Line or Interface section of Configuring IP Services.