The Microsoft DNS server can be either a primary or secondary DNS server to another Microsoft DNS server or to a DNS server running under another operating system (such as UNIX or other vendor's Windows NT implementation).
A primary name server is a DNS server that gets the data for its zones from the local DNS database files. When a change is made to the zone data, such as delegating a portion of the zone to another DNS server or adding hosts in the zone, these changes must be made on the primary DNS server so that the new information is entered in the local zone file.
A secondary name server gets its zone data file from the primary DNS server that is authoritative for that zone. The primary DNS server sends a copy of the zone file to the secondary DNS server in a process referred to as a zone transfer.
The minimum number of DNS servers you need in order to serve each zone is two — a primary and a secondary. Both a primary and a secondary server are required to provide database redundancy and a degree of fault tolerance. Generally, plan to install the primary and secondary servers on different subnets to provide continual support for DNS name queries if one subnet should go down. (Note that Microsoft DNS server automatically makes a zone backup by creating a local DNS backup directory the first time a zone is created on the computer running Microsoft DNS server.)
When a DNS server receives a DNS name query, it attempts to locate the requested information by retrieving data from its local zone files. If this fails because the server is not authoritative for the domain requested and thus does not have the data for the requested domain, the server can communicate with other DNS servers to resolve the request.