Access into and out of the virtual terminal line (vty) ports of the Cisco IOS software can also be controlled
by IP access lists. IOS uses vtys to represent a user who has Telnetted to a router, as well as for Telnet
sessions a user of a router has created to other devices. You can use ACLs to limit the IP hosts that can
Telnet into the router, and you can also limit the hosts to which a user of the router can Telnet.