DNS cache stores the locations (IP addresses) of web servers which contain pages which you have recently viewed. You will be unable to access the site, if the location of the web server changes before the entry in your DNS cache updates!

If you encounter a large number of HTML 404 error codes, and you may need to clear your DNS cache. Once the DNS cache has been cleaned, your computer will query nameservers for the new DNS information.

The following methods allow you to remove old and inaccurate DNS information that may result in 404 errors for MAC and Windows:

Windows® 8

1. Press “Win+X” to open the “WinX Menu”
2. Right-click on “Command Prompt” and select “Run as Administrator”
3. Type the following command and press “Enter”: ipconfig /flushdns
4. If the command was successful, you will see the following message: Windows IP configuration successfully flushed the DNS Resolver Cache

Windows 7

1. Click the “Start” button
2. Enter “cmd” in the Start menu search field
3. Right-click on “Command Prompt” and select “Run as Administrator”
4. Type the following command and press “Enter“: ipconfig /flushdns
5. If the command was successful, you will see the following message: Windows IP configuration successfully flushed the DNS Resolver Cache

Windows XP, 2000, or Vista®

1. Click the “Start” button
2. On the Start menu, click “Run”… 
   1. • If you do not see the Run command in Vista, enter “run” in the Search bar
3. Type the following command in the Run text box: ipconfig /flushdns

MacOS® 10.7 and 10.8

1. Click “Applications“
2. Click “Utilities”
3. Double-click the Terminal application
4. Type the following command: sudo killall -HUP mDNSResponder

Note: To run this command, you will need to know the computer’s Admin account password.

MacOS 10.5 and 10.6

1. Click “Applications“
2. Click “Utilities“
3. Double-click the Terminal application
4. Type the following command: sudo dscacheutil -flushcache

Source: http://docs.cpanel.net