The complete story is like below. While browsing I suddenly went offline and as usual to check the connectivity I ping to glogal DNS address. I got Request Timed Out message. So desperately I tried other open DNS addresses like 18.104.22.168 and 22.214.171.124 but they too failed to get valid reply. Then I just ping www.google.com and it gave me reply and my browser can go to google.com.
Screen shot of my ping operation is attached below.
After the result I again ping 126.96.36.199 and got Request Timed Out message. Mean time I can browse to any website without any issue. I called the ISP and asked what is the actual reason for this issue. After some escalation (Normal procedure :-) )
they told me it was their server mistake and it is corrected now.
Solution to DNS issues from ISPIn case if your ISP's DNS server is not working or not up to full satisfaction we can use open DNS address on your computer. The open DNS addresses are
Steps to assign open DNS in Ubuntu Linux