If you can access some websites but not all of them, here are some possible causes:
Your browser could be attempting to connect through a proxy server. If so, try connecting with the proxy server turned off.
Your browser connection settings could be incorrect. To change connection settings, follow these steps:
Open Internet Options by clicking the Start button , clicking Control Panel, clicking Network and Internet, and then clicking Internet Options.
Click the Connections tab.
Your router could be attempting to proxy DNS requests and failing. Refer to the documentation that came with your router for instructions on disabling DNS proxying.
Parental Controls, Family Safety, or some other parental control software could be blocking access to certain websites.