How to use telnet to check for open ports

Sometimes when installing a new NAV version for a customer I experience problems like a service tier which won’t start. Based on examination of the error in the Application Error log I will then use Telnet to check whether a port is opened or not.

Steps:

  1. First install Telnet. By default this feature isn’t installed. You can accomplish this quick with Powershell:
    Install-WindowsFeature -Name Telnet-Client -Verbose
  2. Now you can open a telnet session. First start a Command Prompt. Now type:
     telnet ipadress port
  3. If you see this then the port is open from the current machine to the IP and port you specified:
    Telnet Port Opened
    If you see this then the port is closed:
    Telnet Port Closed
    In the example above I checked if port 1433 is open on the SQL Server. This way I can verify it isn’t a firewall issue.