The Domain Name System is a hierarchical identification system of domains and subdomains. All domains are, really, subdomains of another domain. At the top of the hierarchy is the (unnamed) Domain Name System itself.
The Domain Name System administers access to the Top Level Domains (TLDs). These are the “extensions” such as: –
Each TLD is administered by a different company or agency, which grants the ability to set up a subdomain:
In many TLD domains, the immediate subdomain is what is purchasable by consumers for use as a domain name. That isn’t always the case, though. Country-code TLDs sometimes have a two-tier system:
Owners of these secondary domain names can, if they choose, create additional subdomains: