well as we know that DNS servers helps you to locate a Web Sites location.. like when you type a site address in ur browser, ur system locates that through a DNS server that which IP address its pointing to and then sends your request to that address...
Every ISP asign you a DNS servers address, like for BSNL it would be 61.0.0.5 ... but some time this DNS servers a hell slow... like for example... I have a domain, domain.com I use it with Web Host A server, so BSNL DNS will point its users to Web Host A addresses... now suppose i change my Host and take the site to Web Host B server or suppose creat one subdomain which has a new IP address, now this has to be updated in BSNL DNS cache to resolve ur systems request to that new server or IP address... but most of the time ISP DNS servers are slow to update these changes...
Here the Public DNS servers comes in... some offers paid service, some are free... these Public servers are open to all, any one can use then, BSNL user , AirTel user... Most of the time they are faster than ISP (specially INdian ISPs)....
an example would be...
http://www.opendns.com/
Off topic..
wah.. Quite a lot of Typeing
public accessible DNS server IP address
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