DNS Flush
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:
10.10.10.10 www.example.com
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.
Windows
ipconfig /flushdns
MAC OSX <= 10.4
lookupd -flushcache
MAC OSX >= 10.4
dscacheutil -flushcache
Ubuntu
sudo /etc/init.d/dns-clean start
Misc Linux
/etc/init.d/nscd restart
service nscd restart