DNS or Domain Name Service is an Internet protocol that helps with the translation of machine based IP addresses into human readable domains like www.domain.com.
When you type a URL into your browser to view a web page, your machine must convert that text to a numeric IP address (something like 220.127.116.11) because machines talk to each other using IP addresses. After you type the domain URL you want, the browser will query that name in the DNS details it currently has. If doesn’t find a match it will then perform a query on a nominated DNS server. This nominated server could be in your network or maintained by an ISP.
If the request again fails to find a matching record the DNS query will be escalated through a hierarchy of DNS servers until a match is found. When a match is found it will be forwarded back through the source DNS servers and finally back to the local machine and the browser.
Once the browser has the ‘resolution’ of the domain name to an IP address it can commence communications with that server and return the web page that the user requested.
As you might now appreciate DNS is a very critical part of the Internet, for without DNS there can be no look ups and most systems wouldn’t be able to communicate with each other. DNS is also a very clever system in that it is hierarchical and distributed. This means that if one DNS server doesn’t have information about the translation of an IP to a domain name then it will query another DNS server. This will continue until a resolution is returned.
When you connect to the Internet you need to tell your machine where an initial DNS server that can be used is. In many cases this is automatically assigned when you login to your machine at work or use the Internet at home. Every time you access the Internet, whether web browsing or sending emails, DNS is working behind the scenes to translate text domain names to machine IP addresses.