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This article applies to version 6.

Other available versions:

Version 5 | Version 4icon.qnmark.png

Available in:

Small Business
Code42 Support

Test your network connection with Telnet

This article applies to version 6.

Other available versions:

Version 5 | Version 4icon.qnmark.png

Available in:

Small Business


Sometimes the Code42 app can't make an active network connection even if the Internet appears to be working. This is because the Code42 app relies on specific ports to be open. Telnet is a great tool for troubleshooting backups to online destinations. This article describes how to use the Telnet client to test connectivity on the correct ports to rule out any sort of issue with firewalls, anti-virus products, or other network issues.

What are ports?

Ports are specific gateways for Internet traffic to travel over. It's like a large hallway with many doors leading outside. If a door is locked, you cannot access the outside world. The Code42 app relies on specific doors to be open. These are different than your Internet browser requires. So if your email and YouTube work, but the Code42 app does not, you should make sure that the proper ports are open.

Before you begin

To use Telnet to troubleshoot a connection to a cloud destination, you need to know the destination's address. If you do not know the address for your destination, contact your administrator.


We are not creating an actual Telnet session, so you should be able to run the test even if Telnet access (TCP 23) is “blocked” on the computer.

Use Telnet

Once you have the address and port, open a command prompt on the source computer.

  1. Install Telnet if it is not already installed. (Telnet is not installed by default on Windows or on macOS High Sierra 10.13.)
  2. Open the command prompt:
    • Windows:
      1. Select Start.
      2. Choose Run or Search.
      3. Enter: cmd.exe
    • Mac: Select Utilities > Terminal.
    • Linux: Open Terminal.
  3. Enter the command: telnet <address of destination port>
    The Code42 app uses port 443 or 4282 to connect to online destinations. If you are unsure which port to include, test both.
    Example: telnet 443

telnet example command

If Telnet successfully connects, the following encrypted connection string (a bunch of unintelligible text) displays:

telnet 443
Connected to
Escape character is '^]'.

If Telnet cannot successfully connect, you may see connection refused, no response, or some other response.

Using telnet to isolate the issue
If the Telnet test passes, there probably isn't an issue with the network. However, some firewall and anti-virus applications are capable of blocking connections on a per-application basis, so make sure the Code42 app has an exception configured in your security software. If the problem continues, contact your administrator.

Install Telnet

Install Telnet on Windows

Telnet is not installed by default in Windows; if you try to run it you will get the message "'Telnet' is not recognized as an operable program or batch file." To install Telnet:

  1. Click Start.
  2. Select Control Panel.
  3. Choose Programs and Features.
  4. Click Turn Windows features on or off.
  5. Select the Telnet Client option.
  6. Click OK.
    A dialog box appears to confirm installation. The telnet command should now be available.

Install Telnet on Macintosh

Telnet is not installed on macOS High Sierra 10.13. To install Telnet:

  1. Paste the following into the Macintosh terminal prompt to install Homebrew, an open-source software package management system:
/usr/bin/ruby -e "$(curl -fsSL"
  1. Run the following command in the terminal prompt to install Telnet:
brew install telnet
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