Sometimes you may need to do a DNS flush, say after a mysterious DNS network issue. Here are the commands.

Mountain Lion / Lion
sudo killall -HUP mDNSResponder

Leopard
sudo dscacheutil -flushcache

Tiger
lookupd -flushcache

Windows
ipconfig /flushdns

Run a nslookup or ping to check if you are indeed getting the correct IP(s) from the DNS server(s).