Sometimes a bad DNS entry will be cached and you will need to either flush (or clear) the DNS cache to get rid of it, or wait up to 24 hours for it to be dropped from the cache automatically.
You can however FORCE the flush from the system avoiding the need to wait 24+ hours!
For Microsoft Windows Users
In Microsoft Windows, you can use the command ipconfig /flushdns to flush the DNS resolver cache.
Click start, go to Run and enter ipconfig /flushdns then hit enter
On somem systems like Windows 7 you’ll need to hold in the ‘Windows Key’ and press R on your keyboard. (Windows key is usually next to ‘alt’)
For Mac OSX Users
In Mac OSX, you can use the command dscachetil -flushcache to flush the DNS resolver cache.
bash-2.05a$ dscacheutil -flushcache
If you have version 10.5.1 or earlier you need to use the following command: lookupd -flushcache