Every now and then you should do a DNS flush on your Mac. Every time you flush the DNS cache, it will clear the DNS information saved/cached on your computer so you will have the lastest data from the root DNS server. This ensures you always the most recent DNS data available to you.
Flush DNS Cache On Mac OSX Lion
If you are using Mac OS X
Lion Leopard (Mac OS X 10.5) , you can simply use the dscacheutil command:
Open Terminal, type the following after the cursor prompt. This will flush the entire cache.
sudo dscacheutil -flushcache
If your Mac OS X version is 10.6 or later. You should use the following instead: ( Thank “algal” for help correct this :))
sudo killall -HUP mDNSResponder
The mDNSResonder will automatically restart cleanly.