Microsoft built the Active Directory entirely around the Domain Name Service or
Domain Name System (DNS) and Lightweight Directory Access Protocol (LDAP).
DNS is the hierarchical replicated naming service on which the Internet is built.
However, although DNS is the backbone directory protocol for one of the largest data
networks, it doesn’t provide enough flexibility to act as an enterprise directory by
itself. That is, DNS is primarily a service for mapping machine names to IP addresses,
which is not enough for a full directory service, which must be able to map names of
arbitrary objects (such as machines and applications) to any kind of information about
those objects.