Every resource record has an associated time-to-live value
or TTL,
which determines how long the data should be considered valid.
A caching name server may
only cache the record for the length of time given by the TTL.
Typical TTL values are on the order of a few days. Many name
servers enforce a minimum TTL of five minutes.
The default TTL value for new records is taken from the
SOA MINIMUM field.