Sorry for not replying back on this .
Actually it was the problem of external firewall. where it seems the entry was made for avira and it was blocking the ip addresses . so, we removed the bridge connection with the router and then it started working fine.
But since then , after every 30-40 mins the internet stops working . so, everytime i have to manually restart the router to fix it.
Maybe this could be an issue with my router . its a D-link DIR-655 router.
And our on call network engineer is saying that its for home purpose, and should be replaced . but dont know if its true