Like many people I know I too had suffered with BSNL broadband DNS issues. The real issue with BSNL DNS is that they constantly changes their DNS IP address without informing their customers. It had in fact caused lot of issues to many BSNL users. Their servers were not so powerful also I guess.
After a long time, accidentally working though DNS servers on my web server, I found a fix for BSNL DNS issue. Instead of using BSNL DNS servers you may use much reliable Google DNS servers. It will instantly resolve your web addresses and is much dependable than BSNL.
You can change your DNS servers to Google’s public DNS servers 18.104.22.168 and 22.214.171.124
Any issue related to BSNL DNS, should not further trouble you. Google uses the same DNS servers worldwide. So it is considered as much fast and reliable than many other DNS servers.