Zones and Records

Zones are an important concept in DNS. A zone is a container that represents a domain on DNS server. The zone contains the records to that domain. There are three types of zones primary, secondary and stub zones.

Every domain, immediately below a TLD, has a zone, such as blogspot.com, but sub-domains, such as scorpits.blogspot.com can be contained within the parent zone or in their own zones. A zone represents a domain in a DNS server, and it contains all of the records of the domain. Sometimes a zone will also contain sub-domains. All zones begin with a SOA record and contain NS records. Zones are typically contained in a zone file, a specially formatted text file that contains all of the records for the zone.

• A Primary zone is the master copy of the zone information; typically you'll only have one primary zone for a domain, but it is possible to have more than one in a multiple master configuration.
• A Secondary zone contains a copy of all of the records in the primary zone; secondary zones are used for redundancy, in case the DNS server containing the primary zone goes down. The secondary zone still contains a copy of the data and can be used for DNS resolution.
• Stub zones only contain name server records, and are used for delegation. When a DNS server is registered as the authoritative DNS server for a domain, stub zones are used to delegate that authority to other DNS servers. 

Those servers will then contain the primary and secondary zones for the domain. Only the primary zone can be edited; when primary zones are edited secondary zones are updated automatically through the zone transfer process, once their time to live expires. 
Zone transfers are used to copy the primary zone to any servers containing corresponding secondary zones. Incremental zone transfers are used where possible, primarily between Microsoft DNS servers. An incremental zone transfer only transfers the data that is changed, since the last zone transfer, so the process is faster and uses less bandwidth. When incremental transfers are not possible, or not supported, full zone transfers are done where the entire zone is transferred each time a change is made to the primary zone.

A sub-domain is set below parent domains; an example of a sub-domain is scorpits.blogspot.com, which resides below the parent domain blogspot.com. Sub-domains can be contained within the parent zone; they can also be contained in their own zone. In addition, through the use of stub zones, sub-domains can reside on other DNS servers. Stub zone indicates that the authority for the zone is delegated to another system.