The Domain Name System (DNS) has a hierarchical inverted tree structure. The DNS hierarchical inverted tree structure is called the DNS namespace. The DNS namespace tree has a unique root and large number of sub-trees. A domain is a subtree of the DNS name space. A Domain can further divided in to sub-domains.
In Domain Name System (DNS) inverted tree structure, top is called the Root. The Root is represented by an empty string "".
After the Root, the next layer in the DNS hierarchy is called as TLDs (Top Level Domains). Examples of TLDs (Top Level Domains) are edu., net., org., com., gov., etc.
The second level in the DNS hierarchy is made up of the business names or organization names. At the second level in the DNS hierarchy, an organization has control over its own name resolution and is authoritative to its own DNS database. Example cisco.com, microsoft.com. omnisecu.com
A Fully Qualified Domain Name (FQDN) identifies the hosts or Sub-domains inside the DNS hierarchy. pc15.tek.omnisecu.com. is a Fully Qualified Domain Name (FQDN), which identifies a computer, under the Sub-domain tek.omnisecu.com. Fully Qualified Domain Name (FQDN) is made up of individual labels separated by dots "." (period).
There are DNS Servers operating at different levels of DNS Namespace hierarchy. ICANN has the overall control of DNS Servers operating at Root Level and Top Level Domains Level. Click the following link view the IP Addresses (IPv4 or IPv6) and Fully Qualified Domain Name (FQDN) of Root Level DNS Servers.
When we register a domain name, (for example omnisecu.com) we control the DNS namespace within omnisecu.com. There is a delegation within within Root Level DNS Servers, pointing to .com. TLD DNS Servers. There is also a delegation within .com TLD DNS Servers, pointing to the DNS servers of omnisecu.com. No other subdomain can be named omnisecu under .com TLD, within the whole DNS namespace.
Now, we control the responsibility of name resolution of omnisecu.com. We can install our own DNS Servers for serving DNS name queries for our domain, omnisecu.com. We can create Resource Records inside the delegated portion of omnisecu.com., create subdomains under omnisecu.com and delegate control of newly created subdomains to other DNS Servers.