Your ISP (Internet Service Provider) automatically assigns primary and secondary DNS Server when your computer requests network information via  DHCP (Dynamic Host Configuration Protocol).

free-dns-servers
If that servers are not reliable or not working properly? There were lot of free and public DNS servers are available. The list is updated on July 2015.

Download from Box.com

We have listed best public DNS Servers of 2015. You can download it from Box.com. Click here to download.

When to change DNS?

If you can’t reach a website with the name, but you can reach with the website with the IP address,  then the DNS server is likely having issues. You have to change the DNS now for better performance.