DNS requests are cached in the local machines for all modern operating systems to allow for faster network performance so if a hostname is requested more than once within a certain amount of time (while the TTL is still good), the machine doesn't have to make a new DNS request. From time to time, DNS caches can become corrupt, or TTL is too high and a change is needed immediately. For these reasons, we need have the ability to flush the cache. The command is different to flush the cache on Leopard than on older versions of OS X. On Leopard, jump to a terminal by launching it from your Dock, browsing to it in the /Applications/Utilities directory in Finder and launching it, or you can bring up spotlight and search for Terminal. Once the Terminal window comes up, you can type the following command on Leopard:

dscacheutil -flushcache

Or on Tiger or older (10.4 or less) you can run:

lookupd -flushcache