Sometime you may like to limit only certain IP address to access your Cisco router / switch by using telnet. You may implement it by configuring the access list and apply it to the line vty 0 4 line.
Further info to my previous post on How to access web page without using web browser, you may use the same method to confirm if a network service is running on a server or computer.
For example if you like to test if pop3 is running on a computer PC1, you may execute the following [...]
Suspend the current Telnet session: Ctrl-Shift-6 x
enter
show sessions: Reveals all sessions are in used.
To Resume the previouly suspended telnet session:
Type the number of the session shown on show sessions