Hi @Vanka Seshagiri Rao ,
It looks like Microsoft no longer offers a download link for the Active Directory Topology Diagrammer (ADTD) tool, you can try contacting the Microsoft support to see if they can provide you with the tool.
http://support.microsoft.com/contactus/?ws=support
You may also use the Internet Archive to download it over here:
http://web.archive.org/web/20200802184044/https://www.microsoft.com/en-us/download/confirmation.aspx?id=13380
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Best regards,
Leon