Telnet protocol allows you to test if a port is open to connections or not using simple telnet command. To run the command, you need to have the telnet client installed.

Steps to enable Telnet client on a Windows PC:

  1. Visit the control panel, and click on 'Programs'
  2. Select 'Turn Windows features on or off'
  3. From the windows feature window, scroll down to 'Telnet Client' 
  4. Enable the client by clicking on the checkbox if it is blank

For a Linux system, you can directly execute the telnet command in the terminal without any installation.

Telnet command syntax

  • telnet <space> <Domain name or IP> <space> <port>
  • eg:

Expected Output

  • If the connection succeeds, there will be no output shown and a blank screen with a blinking cursor will pop up. This indicates that the port is open.
  • Sometimes, an open port will also display a series of messages such as: trying <IP>, connected <domain> and escape character. This also means that the connection is successful for the port.
  • Any other output messages, such as connection failed, operation timeout, access denied and more, indicate that the port is closed.