There is a forgotten file in Windows (Vista) known as a host file and resides in the depths of C Drive at c:\windows\system32\drivers\etc.
The host file has got no file extension but its an ascii file so you can open it in notepad and start editing it because it allows you to rewrite the IPs of hosts that will otherwise resolve correctly by your network connection’s DNS.
On MAC, apparently it’s harder to get there but you can try this (not tested):
Enter the Terminal (Applications / Utility). Type this:
sudo /Applications/TextEdit.app/Contents/MacOS/TextEdit /etc/hosts
With a little bit of imagination, this bit of information will become very useful. As to what use, I will not suggest here.