Primary & Secondary DNS

Primary DNS servers are authoritiative for a particular domain.  Secondary DNS servers are also authoritative for a domain, although they are simply replicas of the Primary DNS server. 

Primary and Secondary DNS is a master-slave relationship.  The Master DNS server is the "primary" while the secondaries are the "slaves".

Many people confuse this concept with the primary and secondary nameservers defined for your domain registration.  When you define two or more servers in the domain registration as nameservers, the DNS system has little or no concept of which one of those servers is the primary.  It will use all of the servers defined equally in round-robin fashion.  The more servers you have, the greater the reliability.  If a server is down, the client will try a different server.

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