Can a server block requests for a website from network syste

hi friends ,
if it possible to block or allow access from server while taking websites from network system on same network.
i need to block the websites on network  from server ... .
i need to write c# code to put on server to check the requesting website from local network system are allowed or not if it is allowed then they are accesssible thr server 
please help ...
Advance thanks for helping me ...