OS X stores a local cache of resolved DNS queries for a time defined by the DNS server, but there are times when it may be necessary to reset the cache immediately and re-query a DNS server. For example, you might do this after a change to an entry on the server or a new entry is added.
OS X Mountain Lion or Lion
Use the following command in the Terminal to reset the DNS cache:
sudo killall -HUP mDNSResponder
Mac OS X v10.6
Use the following command in the Terminal to reset the DNS cache:
sudo dscacheutil -flushcache
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