To understand the DNS mechanism, it is important to first understand the domain name space. The domain name space is a “tree” of nodes. The easiest analogy is that it is like a “tree” of directories (folders) in a file system.
A DNS zone is an administrative entity that assumes administrative control over a portion of the domain name space (tree). A special zone called the “root zone” is at the top level. The “root zone” is denoted by a single period (.). This zone is authoritative over the top level domains, such as com, org, edu, ca, us and etc. The period of the root zone is often omitted because it is common to all domain names. Nonetheless, domains will resolve correctly with a trailing period to represent the root zone!
Note that the “root zone” is responsible for the entire domain name space. It is managed by ICANN (Internet Corporation for Assigned Names and Numbers).
The root zone delegates the administration of portions of the domain name space to other DNS zones. In our example, for example, a DNS zone may be responsible for google and all subdomains thereof. Note that the delegation of administration of domain names may not be one level at a time. For example, the DNS zone of google.com may be responsible for domain names like ad.maps.google.com, or any other domain names with the suffix of google.com.