The Host request-header field specifies the Internet host and port
number of the resource being requested, as obtained from the original
URI given by the user or referring resource (generally an HTTP URL,
as described in section 3.2.2). The Host field value MUST represent
the naming authority of the origin server or gateway given by the
original URL. This allows the origin server or gateway to
differentiate between internally-ambiguous URLs, such as the root "/"
URL of a server for multiple host names on a single IP address.
另外，RFC-2616并非HTTP协议的最新规范，具体的规范请参考RFC-2616的描述链接：Information on RFC 2616