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How to Test TCP Connection?

Published in Network Troubleshooting 2 mins read

You can test a TCP connection using the Telnet command-line tool, which is available on most operating systems.

Using Telnet to Test TCP Connection

Telnet allows you to attempt a connection to a specific IP address or hostname on a given port. This helps determine if a service is listening on that port and if the connection is allowed by any firewalls.

Steps to Test TCP Connection Using Telnet:

  1. Open Command Prompt (Windows): Press the Windows key + R, type "cmd.exe", and click OK.

  2. Enter the Telnet Command: Use the following syntax:

    telnet <IP address or hostname> <port number>
    • Replace <IP address or hostname> with the actual IP address (e.g., 10.17.xxx.xxx) or hostname (e.g., www.example.com) of the server you want to test.
    • Replace <port number> with the TCP port number you want to test (e.g., 5000 or 1723).

  3. Example: To test the TCP port 5000 on the IP address 10.17.xxx.xxx, you would enter:

    telnet 10.17.xxx.xxx 5000

    To test port 1723 on the hostname www.example.com, you would enter:

    telnet www.example.com 1723

  4. Interpret the Results:

    • Successful Connection: If the Telnet connection is successful, you will see a blank command prompt window or a connection-specific message. This indicates that the TCP port is open and listening.
    • Connection Failed: If the connection fails, you will receive an error message like "Could not open connection to the host" or "Connection refused." This indicates that either the port is not open, the service is not running, or a firewall is blocking the connection.

Important Considerations:

  • Telnet Client Installation: On some operating systems (like recent versions of Windows), the Telnet client may not be enabled by default. You may need to enable it through the Control Panel's "Turn Windows features on or off" settings.

  • Security: Telnet transmits data in plain text, so it's not secure for transmitting sensitive information. It's primarily useful for basic connectivity testing. For secure connections, consider using tools like ssh or openssl.

  • Alternative Tools: netcat (nc) is another powerful command-line tool for testing TCP connections. It offers more flexibility and features than Telnet, though it might require separate installation.

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