How to flush the DNS cache:

Choose System Clean > Flush DNS Cache.
Every website such as google.com has an underlying server address called the IP address. To load your websites faster, Mac OS X stores the IP address for websites in the DNS cache.
You may want to empty that DNS cache for a number of reasons, such as:
Privacy. If you want to remove traces of your visited websites. You should also empty your browser cache and history in this case.
Network administration. You’re a web or network administrator that just changed the DNS entries of one of your servers (that is, get rid of “stale” IP addresses).
Choose System Clean > Flush DNS Cache.
Tip: if you want to empty your DNS cache for privacy reasons, you may also want to empty your browser cache and browser history.