What is the dns hierarchical namespace?

[mountaing]

Wikipedia link for DNS: https://en.wikipedia.org/wiki/Domain_Name_System

Following the letters A - E along side the diagram.

A

The Root Name Server, stated stated as a dot. This is where unknown name to IP start. https://en.wikipedia.org/wiki/Root_name_server

B

The Top-Level Domain (TLD), contains many types as explained in the wikipedia link: https://en.wikipedia.org/wiki/Top-level_domain

C

Second-Level Domain, at this level most users are familiar as the names of companies, academic institutions, organizations, and other names under the TLD. e.g. hp is a second-level domain of TLD .com https://en.wikipedia.org/wiki/Second-level_domain

D

Third-Level Domains and lower. The hierarchy continues.

The URI in DNS is read right to left, starting with the dot, Root Name Server, to fully qualify the domain name. The forth-level domain in the diagram is just random name to demonstrate you can other sub-domains.