Presentation
In this tutorial, we will see how to use the Telnet client to test the connection to a service (IP: PORT) for diagnostic purposes.
Telnet is a client / server TCP protocol that listens on port 23 which allows the administration of equipment (switches, firewall …) and of certain servers (UNIX, LINUX).
It is also possible to use the Telnet client to test the communication with a service (SMTP, HTTP …), which we will see now, this to allow you to make sure that the network protocol of an application is working.
By default on Windows the Telnet client is not installed, a tutorial on installing the telnet client on Windows Server is available here.
Test a connection to a port with a Telnet client
To illustrate this tutorial, we will test the connection to google.fr on port 443 and on exemple.priv 443 which does not exist to compare the result.
Open a command prompt.
In the command prompt enter the command below, adapting to the situation
telnet <ip|fqdn> port
telnet google.fr 443 
telnet exemple.priv 443
On the screenshots below, we can see the result of the connection, on the right image the connection to google.fr where we are left with a black screen where it is possible to place orders if the service l ‘accepts and on the image on the left, an error message appears indicating that the connection could not be established.
Connexion établie 
Erreur de connexion
You now know how to use the Windows Telnet client to test the functioning of a network service.
Ressource :
Use Telnet on Port 25 to test SMTP communication (Microsoft).