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I came across this problem when I tried to click the Telnet link in CiscoWorks Device Center using Internet Explorer 8 and got a rather unwelcome error.

It turns out a change in security policies for Internet Explorer 7 onwards disables telnet URLs, URLs beginning telnet:// which invoke the users telnet client.

Here are details of how you can re-enable this feature by editing the Windows registry. I’ve only tested this change on Windows XP SP3, but my understanding is that it works on all revisions of XP.

Important This section, method, or task contains steps that tell you how to modify the registry. However, serious problems might occur if you modify the registry incorrectly. Therefore, make sure that you follow these steps carefully. For added protection, back up the registry before you modify it. Then, you can restore the registry if a problem occurs. For more information about how to back up and restore the registry, click the following article number to view the article in the Microsoft Knowledge Base:

322756 (http://support.microsoft.com/kb/322756/ ) How to back up and restore the registry in Windows
  1. Run up regedit and add the following key:
    HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Internet Explorer\Main\FeatureControl\FEATURE_DISABLE_TELNET_PROTOCOL
  2. Add a DWORD within the new key, name the DWORD iexplore.exe and set its value to 0
  3. Close all instances of Internet Explorer and open a new browser window.