Vista clients are unable to join the domain with numeric top level domain name

Source: Microsoft Support

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You are unable to join a Windows Vista computer to an Active Directory domain. This happens when the top level domain (TLD) name ends in all numeric characters, for example, mydomain.123. If you PING this name (ping mydomain.123) from the Vista computer, the name is not resolved to the IP address. However, if you PING this name appending with the "." (ping mydomain.123.), the name is resolved.

You may work around this issue by skipping the TLD check. To do this, set the ScreenBadTlds registry value.

1. Run Registry Editor (regedit.exe) and browse to the following registry key:

[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Dnscache\Parameters]

2. In the right pane, create the following value if it does not exist:

Value: ScreenBadTlds
Type: REG_DWORD

3. Double click the ScreenBadTlds value, and sets its data to 0 (0x00000000).

4. You can try to join the Windows Vista computer to the domain again.

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