Domain Name System (DNS) is a name resolution method that is used to resolve host names to IP addresses. It is used on TCP/IP networks and across the internet. DNS is a namespace. Active Directory is built on DNS. DNS namespace is used internet wide while the Active Directory namespace is used across a private network. The reason behind the choice of DNS is that it is highly scalable and it is an internet standard.
In case of Active Directory, DNS maintains a database of services that are running on that network. The list of services running are maintained in the form of service records (SRV). Service records allow a client in an active directory environment to locate any service it needs such as a printer. These SRV records are used to identify the domain controllers also.
A single DNS server cannot help in resolving a resource record. Several DNS servers are used in the process.Each DNS server queries its own database to find an address corresponding to a record. If the requested information is not available, then it forwards the query to another DNS server. For example, a name resolution may first query an Internet root server, then the first–level domain server, and then the second–level domain server and so on to resolve the name to its associated address.
Every time the computer’s IP address changes, making manual entries into the DNS database is time consuming and might result in some entries being left out. Hence Dynamic DNS is required to make these updates automatic. Any newly installed server can also automatically register its IP address and SRV records with the DNS server. Active Directory supports such Dynamic updates to be made.