完全限定域名（FQDN）包含通过 DNS 系统明确地按名称查找此“权威”的所有必要部分。
For example, in "developer.mozilla.org":
- "org" is called a top-level domain. They are registered as an internet standard by the IANA . Here, "org" means "organization" which is defined in a top-level domain registry.
- "mozilla" is the domain. If you like to own a domain you have to register it with one of the many registrars who are allowed to do so with a top-level domain registry.
- "developer" is a "sub-domain", something you as the owner of a domain may define yourself. Many owners choose to have a subdomain "www" to point to their World_Wide_Web resource, but that's not required (and has even fallen somewhat out of favor).
- 在维基百科中的 域