Windows authentication with Active Directory

i created a website that currently uses windows authentication. user opens the webiste and NT login and password are checked behind the scenes. currently i have a database with a table called Admins and each user with their role is stored in this table. The tool checks to make sure that the NT Username matches the one in the table and then opens the website. the customer wants to change this security to be in active directory. I am new to all of this still, learning as i go. I would like to know if anyone can assist me with what i need to do to get this accomplished per customers request.

 let me know if i have not provided you with enough information.

 i am using ASP.NET with C#

 

Thanks.
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