Caching and time to live
Because of the huge volume of requests generated by a system like the DNS, the designers wished to provide a mechanism to reduce the load on individual DNS servers. The mechanism devised provided that when a DNS resolver (i.e. client) received a DNS response, it would cache that response for a given period of time. A value (set by the administrator of the DNS server handing out the response) called the time to live, or TTL defines that period of time. Once a response goes into cache, the resolver will consult its cached (stored) answer; only when the TTL expires (or when an administrator manually flushes the response from the resolver's memory) will the resolver contact the DNS server for the same information.
Generally, the time to live is specified in the Start of Authority (SOA) record. SOA parameters are:
Read more http://blogmee.info/index.php/what-does-serial-refresh-retry-expire-minimum-and-ttl-mean/
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