Edit

"Error 633 - the modem (or other connecting device) is already in use" error message when you try to establish a VPN connection

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This article provides a workaround for the issue where you may not be able to establish a VPN connection because of a TCP port conflict.

Applies to:   Windows Server 2003
Original KB number:   885959

Important

This article contains information about modifying the registry. Before you modify the registry, make sure to back it up and make sure that you understand how to restore the registry if a problem occurs. For information about how to back up, restore, and edit the registry, click the following article number to view the article in the Microsoft Knowledge Base:
256986 Description of the Microsoft Windows Registry

Symptoms

When you try to use a virtual private network (VPN) connection on a Microsoft Windows Server 2003-based computer, you may receive the following error message:

Error 633 - the modem (or other connecting device) is already in use or is not configured properly.

Cause

This issue may occur if the TCP port that is used by the VPN connection is already being used by another program. The VPN connection uses TCP port 1723 to establish a connection. If another program is already using this port, you cannot establish a VPN connection.

Workaround

Warning

If you use Registry Editor incorrectly, you may cause serious problems that may require you to reinstall your operating system. Microsoft cannot guarantee that you can solve problems that result from using Registry Editor incorrectly. Use Registry Editor at your own risk.

To work around this issue, use one of the following methods:

  • Explicitly reserve the TCP port that is used for the VPN connection. To do it, follow these steps:
    1. Click Start, click Run, type regedit.exe in the Open box, and then click OK.
    2. In Registry Editor, locate and then click the following registry subkey: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters
    3. On the Edit menu, point to New, and then click Multi-String Value.
    4. Rename the multi-string value as "ReservedPorts," and then double-click ReservedPorts.
    5. In the Value data box, type 1723-1723, and then click OK.
    6. Quit Registry Editor.
  • Use the netstat command to find the program that uses port 1723. Then, end the process for that program. To do it, follow these steps:

    1. Click Start, click Run, type cmd.exe in the Open box, and then click OK.

    2. At the command prompt, type the following command, and then press ENTER:
      netstat -aon

    3. In the output that is displayed, identify the process ID for the program, if any, that uses TCP port 1723.

    4. At the command prompt, type the following command, and then press ENTER:
      taskkill /PID PID /F

      Note

      The taskkill command ends the process that corresponds to the process ID number. The /F option is used to forcefully end the process.

    5. At the command prompt, type exit, and then press ENTER to quit the command prompt.

  • Restart your computer. It may cause the program that uses TCP port 1723 to use a different port.