Why i can't open some site in my laptop?

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I do work so many ptc sites and i always looking for best ptc site so always join new ptc. But i can't open so many sites in my laptop?Why and what does it means? It shows server not found or problem loading page. Moreover i did work some site but recently when i try to open that site it also show the same. In this situation what should i do? How can i use all the site to my laptop? Thanks in advance!