Section 1: Telnet
Before we learn about SSH (Secure Shell), we need to talk about Telnet.VTY (Virtual Teletype) lines are used to let you access and control a router or switch 
from a distance(remotely).It also allows remote access to 
devices, but SSH encrypts the data, keeping it secure from hackers.Like Telnet, SSH requires a client and server, but it uses TCP port 22.The big problem with Telnet is that it sends everything in plain text, including 
usernames and passwords.SSH (Secure Shell) is a safer replacement for Telnet.Most Cisco devices have 5 VTY lines (some have 16), numbered from 0 to 4.In short, VTY lines are important for remote access either Telnet or SSH, and 
management of network devices.Telnet uses TCP port 23 by default.