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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.

Section 1: Telnet
Before we learn about SSH (Secure Shell), we need to talk about Telnet. Telnet is a
protocol that lets you communicate with a remote device over a network.
It is mostly used by network administrators to access and manage devices from far
away.
To use Telnet, you need a Telnet client installed on your computer, and the remote
device needs a Telnet server. Telnet uses TCP port 23 by default.
The big problem with Telnet is that it sends everything in plain text, including
usernames and passwords. This makes it unsafe, as anyone could see your
information. Because of this, Telnet is rarely used today.
SSH (Secure Shell) is a safer replacement for Telnet. 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 encryption
used by SSH makes it the preferred choice for network security.
Section 1.1: Telnet Configuration
Before we start the configuration process, we must know an important topic which is
called VTY lines.
VTY (Virtual Teletype) lines are used to let you access and control a router or switch
from a distance(remotely).
They are like doors that allow someone to log in to the device using programs like
Telnet or SSH.
Most Cisco devices have 5 VTY lines (some have 16), numbered from 0 to 4.
These lines allow multiple people to connect at the same time.
In short, VTY lines are important for remote access either Telnet or SSH, and
management of network devices.