Like the UNIX file system, DNS domains are organized as a set of descending branches similar to the roots of a tree. Each branch is a domain, each subbranch is a subdomain. The terms domain and subdomain are relative. A given domain is a subdomain relative to those domains above it in the hierarchy, and a parent domain to the subdomains below it.
For example, in Figure 5–1, com is a parent domain to the Acme, Ajax, and AAA domains. Or you could just as easily say that those are subdomains relative to the com domain. In its turn, the Ajax domain is a parent to four subdomains (Sales, Manf, QA, and Corp).
A domain contains one parent (or top) domain plus the associated subdomains if any. Domains are named up the tree starting with the lowest (deepest) subdomain and ending with the root domain.