The DNS namespace is based on a hierarchical tree structure, as presented in the following diagram. The structure is as follows:

• The network of root servers (http://www.iana.org/domains/root/servers).
• The network of Top-Level Domain (TLD) servers (http://www.iana.org/domains/root/db).
• Each top-level domain has name servers similar to that of IANA administers. Top-level domains contain second-level domains. TLDs are the highest-level servers, for example, country servers, as illustrated in the following diagram.
• Second-Level Domain (SLD) contains the domains and names for organizations and countries. The names in second-level domains are administered by the organization or country specified:

There are some important definitions, as shown in the following diagram:

• Domain: Constitutes all branches under ndi-com.com, in this case an SLD
• Zone: A contiguous portion of a DNS domain in the DNS namespace, whose database records exist and are managed in a particular DNS database file stored on one or multiple DNS servers: