This is the most commonly used method of restricting Telnet access to Cisco IOS-based Routers and Switches.

1. Connect to your Cisco switch running IOS. This will be via console if you are configuring the router/switch for the first time.
2. 
   
3. Type "enable" and press Enter to go to "enable" mode.
4. Type "config t" and press Enter to go to "configure" mode.
5. Type "line vty 0 4" and press Enter to go into line configuration mode.
6. Type "login" and press Enter to configure the router/switch to present a password prompt when a user telnets to the device.
7. Type password yourpassword" to configure the password you want to be used to access the router/switch.
8. Type "exit" and press enter. Then Type "exit" and press enter again to get back to enable mode.
9. Telnet to the router/switch and enter the password you configured above at the prompt., then press Enter.
   
   
   Only after a successful login should you go to the next step!
   
10. Type "wri mem" and press Enter to save your configuration.