From Lunarpages Web Hosting Wiki
Flushing your DNS settings can sometimes help troubleshoot domain name issues. This is how you flush your DNS settings via the various PC operating systems out there.
For Mac OSX, you can use the command dnscacheutil -flushcache to flush the DNS resolver cache:
bash-2.05a$ dnscacheutil -flushcache
For Mac OSX versions 10.5.1 and before, the command lookupd -flushcache performed the same task:
bash-2.05a$ lookupd -flushcache
Windows XP and Vista
Click on Start, Run and then type in cmd to launch the Command Window. From there, type in:
On Windows Vista, you will need to click on the Windows logo, then All Programs, Accessories and right-click on Command Prompt. Then select, "Run as Administrator" and type in the above command.
For Linux, the nscd daemon manages the DNS cache. To flush the DNS cache, restart the nscd daemon by using the command: