CISCO IOS, allow to connect via console, direct RS232 cable to the switch/router, telnet, or sometimes ssh.
* line 0 = Direct RS232 local console via serial cable
* line vty 0 4 = 5 first TCP/IP connection (Telnet or SSH)
* line vty 5 15 = 10 next TCP/IP connection (Telnet or SSH)
You can, for safety disable the telnet, by allowing only SSH via : transport input ssh
This only if it is allowed by your IOS :
CISCO IOS can be powere by a so-called k8 or k9 version which means :
* k9 = Strong AES/3DES encryption
* k8 = Weak DES encryption