在一些服務器的默認配置中，這個內容頭部將引用一個IP位址，或者可能提供也可能不提供完全限定域名(FQDN)。
FQDN (Fully Qualified Domain Name)
In a default configuration, in some Web servers, this content header will reference an IP address and may or may not provide the fully qualified domain name (FQDN).
Sometimes an address in this form is called a fully-qualified domain name or FQDN.
