As airtel users might be knowing that when you enter a wrong url then you are redirected to advertisement filled pages of airtel, this is kind of unwanted redirection by airtel for their own profit and it is official according to this page http://airtel.in/wps/wcm/connect/Airtel.in/airtel.in/home/whats+new/change-your-dns
This might cause some problems with download programs or your searches on the web. To bypass the airtel's DNS servers you can either use openDNS or google's DNS service.
preferred dns : 184.108.40.206
alternate dns: 220.127.116.11
preferred dns : 18.104.22.168
alternate dns : 22.214.171.124