Disclaimer:
Before installing and using Telnet, keep the following in mind.

Using Telnet in public network(WAN) is very very bad idea. It transmits login data in the clear format. Everything will be sent in plain text.
If you still need Telnet, It is highly recommended use it in the local area network only.
Alternatively, you can use SSH. But make sure you’ve disabled root login in SSH.
What Is Telnet?
Telnet is a network protocol which is used to connect to remote computers over TCP/IP network. Once you establish a connection to the remote computer, it becomes a virtual terminal and will allow you to communicate with the remote host from your local system.

In this brief tutorial, let us see how to install Telnet, and how to access remote systems via Telnet.

Installation
Open your terminal and type the following command to install telnet:

yum install telnet telnet-server -y
Now, the telnet has been installed in your server. Next, edit the telnet configuration file /etc/xinetd.d/telnet;

vi /etc/xinetd.d/telnet
Set disable = no:

# default: on
# description: The telnet server serves telnet sessions; it uses \
#       unencrypted username/password pairs for authentication.
service telnet
{
        flags           = REUSE
        socket_type     = stream
        wait            = no
        user            = root
        server          = /usr/sbin/in.telnetd
        log_on_failure  += USERID
        disable         = no
}
Save and quit the file. Be mindful that you don’t have do this step in CentOS 7.

Now restart the telnet service using the following command:

On CentOS 6.x systems:

service xinetd start
Make this service to start automatically on every reboot:

On CentOS 6:

chkconfig telnet on
chkconfig xinetd on
On CentOS 7:

systemctl start telnet.socket
systemctl enable telnet.socket
Allow the telnet default port 23 through your firewall and Router. To allow the telnet port through firewall, Edit file /etc/sysconfig/iptables on CentOS 6.x systems:

vi /etc/sysconfig/iptables
Add the line as shown in red color:

# Firewall configuration written by system-config-firewall
# Manual customization of this file is not recommended.
*filter
:INPUT ACCEPT [0:0]
:FORWARD ACCEPT [0:0]
:OUTPUT ACCEPT [0:0]
-A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
-A INPUT -p icmp -j ACCEPT
-A INPUT -i lo -j ACCEPT
-A INPUT -p tcp -m state --state NEW --dport 23 -j ACCEPT
-A INPUT -m state --state NEW -m tcp -p tcp --dport 22 -j ACCEPT
-A INPUT -j REJECT --reject-with icmp-host-prohibited
-A FORWARD -j REJECT --reject-with icmp-host-prohibited
COMMIT
Save and exit the file. Restart iptables service:

service iptables restart
On CentOS 7, run the following commands to enable telnet service through firewall.

firewall-cmd --permanent --add-port=23/tcp
firewall-cmd --reload
Thats it. Now telnet server is ready to use.

Creating users
Create a test user, for example “sk” with password “centos“:

useradd sk
passwd sk
Client Side Configuration
Install telnet package:

yum install telnet
On DEB based systems:

sudo apt-get install telnet
Now, open Terminal, and try to access your server(remote host).

If your client is Linux system, open the terminal and type the following command to connect to telnet server.

telnet 192.168.1.150
Enter username and password which we have created in the server:

Sample output:

Trying 192.168.1.150...
Connected to 192.168.1.150.
Escape character is '^]'.

Kernel 3.10.0-123.13.2.el7.x86_64 on an x86_64
server1 login: sk
Password:
[sk@server1 ~]$
As you see in the above output, the remote system has been successfully accessed from the local machine.

If your client is windows system, then go to Start -> Run -> Command Prompt.

In the command prompt, type the command:

telnet 192.168.1.150
Where 192.168.1.150 is remote host IP address.

Now you will be able to connect to your server.

That’s it.

Cheers!