1. It is happening with URLs ending with .co.in and .nic.in - I can still reach the sites if I use their IP address.
2. If the site has a pop-up secondary secure area (such as banking sites) to be reached by logging into a secure server with the same IP, I am unable to reach that login page, because the pop-up page would not load and show a DNS error.
3. I checked my hosts file thoroughly, and could not find any mention of these IPs or corresponding URLs.
4. The problem is not occurring if I use the same URL in my wife's Mac Safari browser (though connected to the same network).
I am going nuts over this problem, because I could not find any apparent reason for its occurrence. I would be obliged if someone could shed some light on this.