The Macintosh operating system and most computer operating systems for that matter will cache DNS requests. That is a good thing as it speeds up the DNS lookups within the same host. But sometimes this can become problematic when the cache contains values that are no longer valid or legitimate. Also nefarious software, viruses, or spyware can poison a DNS cache. Thus it can be necessary to clear (or reset) the DNS cache on your Mac and  to do that in Mac (OS Tiger and below) you open the Terminal.app program usually found within your Applications/Utilities/ folder and you type the following command:

lookupd -flushcache

However a Mac OSX 10.5 Leopard user will tell you this command did not work on their terminal. In Leopard a new command has been used to replace flushcache, so to clear DNScache in Leopard, just type this in your Terminal and hit enter.

dscacheutil -flushcache