Most (if not all) modern operating systems keep a local DNS cache some are pretty good at keeping them updated some aren't. Either way from time to time when making DNS changes it is useful to know how to clear your DNS cache. This is just a quick reference of how to clear DNS cache on several popular operating systems.
Mac OS X 10.5 and after (leopard)
Mac OS X 10.4 and before (tiger)
Depending on your distro and configuration one of the following should work fine. Don't worry regardless of your setup all of these command should be safe they just might not do anything.
service nscd restart
nscd -i hosts