A subdomain is a domain located beneath another domain in the hierarchical Domain Name System (DNS). Domains form a tree: the root domain contains top-level domains (TLDs), TLDs contain second-level domains (SLDs), and each lower-level domain can contain further subdomains. section 3.1.3
Read subdomains from right to left in a fully qualified domain name (FQDN):
- In google.com, google is beneath the com TLD and is therefore a subdomain of com. section 3.1.3
- In www.google.com, www identifies a lower-level name beneath google; www is a conventional website label, not a requirement. section 3.1.3
- The final dot in www.google.com. represents the root domain; it is commonly omitted. section 3.1.3
Hierarchy example
The path . → com → google → www shows that google is a subdomain of com, while www is a subdomain of google. section 3.1.3
Figure 3.3 The DNS hierarchy. All domains are under the root domain, represented by “.”. Under the root domain, there are various top-level domains. Under each top-level domain, there are various second-level domains. The “google” domain, a subtree under the “com” top-level domain, is highlighted.
Use the hierarchy diagram to trace the root domain, TLDs, SLDs, and lower-level subdomains. section 3.1.3
