Sometimes it’s necessary to have two DNS servers that are authoritative for the same DNS namespace. This is the case if you use the same namespace for your web site and your internal Active Directory domain, e.g. terlisten-consulting.de. Or that you have created the zone terlisten-consulting.de in your Windows DNS to point specific hosts to internal IP addresses. The DNS servers at your ISP would be authoritative, and the domain controllers of your Active Directory would also be authoritative for the same domain.