2.4 DNS Structure

1.	The DNS uses a large number of servers, organized in a hierarchy and distributed around the world.
2.	Thus, hostname-IP mappings are distributed among such servers rather than stored on centralized server.
3.	Not using the centralized server approach avoids the following problems:

		A single point of failure.
		Huge traffic volumes would require handling by a single server.
		Hosts will be geographically far from the central server.
		Continuous updates are required on the server to add records of each new host added.

4.	There are three main types of DNS servers root servers, top level domain servers and authoritative servers which are organized in a hierarchy as shown in Figure 2.1:

a)

Root DNS servers: There are 13 root DNS servers (labeled a to m) on the Internet, most of which are located in North America.

2.4 DNS Structure

			More details on the root DNS servers can be found at http://www.root-servers.org. Each of the servers is actually a cluster of server to improve reliability and security.
			Root DNS servers contain the IP addresses of all TLD servers.

b)

Top-Level Domain (TLD) servers: responsible for top-level domains such as .com, .org, .net, .edu, .gov and all of the country top-level domains such as .my, .ps, .uk, .fr and .jp.

			The Network Solutions company maintains the TLD servers for the .com domain and the Educause company maintains the TLD servers for the edu domain.
			TLD servers contains records of the IP addresses for the authoritative DNS servers under a specific top level domain.

c)

Authoritative DNS servers: Each organization that has publicly accessible hosts on the Internet must have an authoritative DNS server containing the mapping between the hostnames of its hosts to their IP addresses.

Such mapping can alternatively be stored on the authoritative DNS servers of Internet Service Providers (ISPs).

2.4 DNS Structure

Figure 2.1: Hierarchical Structure of DNS