The complete Domain Name System (DNS) hierarchy cannot be stored on a single server. Portions of Domain Name System (DNS) hierarchy is divided and stored among multiple DNS servers.
A DNS (Domain Name System) Zone is a database that contains Resource Records of a contiguous DNS Namespace. The administrative responsibility for a DNS Namespace has been delegated to an organization, by using the concept of DNS Zone. The DNS Zone is Authoritative for the portion of DNS Namespace which it holds.
For Example: If we have a zone called omnisecu.com inside a DNS Server, we can create Resource Records for all the TCP/IP devices inside onmisecu.com domain under the zone. This DNS Server is delegated at its higher level DNS Server (.com level) to handle all the DNS Queries under omnisecu.com, like www.omnisecu.com, tek.omnisecu.com, serv-15.omnisecu.com etc.
DNS Zones can be generally classified into two types. 1) Primary DNS Zone 2) Secondary DNS Zone.
Primary DNS Zone: A Primary DNS zone is the original Read-Write Authoritative DNS zone of portion of a DNS Namespace. When a DNS Server hosts a primary zone, that DNS Server is considered as the Authoritative DNS Server and it is the primary source for information of that zone. Zone updates are possible only in a Primary DNS zone. Primary DNS zone is hosted in the Primary DNS Server.
Secondary DNS Zone: A Secondary DNS Zone is Read-Only copy of a Primary Zone, or another Secondary Zone, kept in a Secondary DNS Server. A Secondary DNS Zone is used to reduce the load on Primary DNS Servers and also for preventing single point of failure.
The Zone information from the Primary DNS Server is transferred to the Secondary DNS Server via a process known as Zone Transfer.