Suppose the host cis.poly.edu desires the IP address of gaia.cs.umass.edu. Also suppose that Polytechnic’s local DNS server is called dns.poly.edu and that an authoritative DNS server for gaia.cs.umass.edu is called dns.umass.edu. As shown in the video, the host cis.poly.edu first sends a DNS query message to its local DNS server, dns.poly.edu. The query message contains the hostname to be translated, namely, gaia.cs.umass.edu. The local DNS server forwards the query message to a root DNS server. The root DNS server takes note of the edu suffix and returns to the local DNS server a list of IP addresses for TLD servers responsible for edu. The local DNS server then resends the query message to one of these TLD servers. The TLD server takes note of the umass.edu suffix and responds with the IP address of the authoritative DNS server for the University of Massachusetts, namely, dns.umass.edu. Finally, the local DNS server resends the query message directly to dns.umass.edu, which responds with the IP address of gaia.cs.umass.edu.

Three classes of DNS servers: 

• Root DNS servers. In the Internet there are 13 root DNS servers (labeled A through M), most of which are located in North America. Although the 13 root DNS servers are referred as if it were a single server, each “server” is actually a network of replicated servers, for both security and reliability purposes. All together, there are 247 root servers as of fall 2011. 

• Top-level domain (TLD) servers. These servers are responsible for top-level domains such as com, org, net, edu, and gov, and all of the country top-level domains such as uk, fr, ca, and jp. The company Verisign Global Registry Services maintains the TLD servers for the com top-level domain, and the company Educause maintains the TLD servers for the edu top-level domain.  

• Authoritative DNS servers. Every organization with publicly accessible hosts (such as Web servers and mail servers) on the Internet must provide publicly accessible DNS records that map the names of those hosts to IP addresses. An organization’s authoritative DNS server houses these DNS records. 

There is another important type of DNS server called the local DNS server. A local DNS server does not strictly belong to the hierarchy of servers but is nevertheless central to the DNS architecture. Each ISP—such as a university, an academic department, an employee’s company, or a residential ISP—has a local DNS server (also called a default name server). When a host connects to an ISP, the ISP provides the host with the IP addresses of one or more of its local DNS server.

Resourse:https://media.pearsoncmg.com/bc/abp/cs-resources/products/series.html#series,series=Kurose

Creative graphics and animations are included to provide concept of the DNS query.