When your surfing the web your system caches the location of all websites it comes across by saving the IP address and domain name. For example:
While is great for reducing overhead by skipping the DNS lookup; problems arise when the website changes it's IP. If your ever having trouble getting to a website you know is there, you can always try running one of these commands to flush the DNS cache.
MAC OSX <= 10.4
MAC OSX >= 10.4
sudo /etc/init.d/dns-clean start
/etc/init.d/nscd restart service nscd restart