Active Directory Root Name

Hello
 I want to retrieve data from Active Directory but I am not sure how can I get the root name available in my Active Directory. I know that if I don't give any root name while creating DirectoryEntry object i.e
                         DirectoryEntry root = new DirectoryEntry("");
then it will start from top most node of the ActiveDirectory tree but if i want to start from some where in mid what should be the root name ?
How can i get all root names from Active Directory ?

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