The exact resource identified by an Internet request is determined by
examining both the Request-URI and the Host header field.
An origin server that does not allow resources to differ by the
requested host MAY ignore the Host header field value when
determining the resource identified by an HTTP/1.1 request. (But see
section 19.6.1.1 for other requirements on Host support in HTTP/1.1.)
An origin server that does differentiate resources based on the host
requested (sometimes referred to as virtual hosts or vanity host
names) MUST use the following rules for determining the requested
resource on an HTTP/1.1 request:
1. If Request-URI is an absoluteURI, the host is part of the
Request-URI. Any Host header field value in the request MUST be
ignored.
2. If the Request-URI is not an absoluteURI, and the request includes
a Host header field, the host is determined by the Host header
field value.
3. If the host as determined by rule 1 or 2 is not a valid host on
the server, the response MUST be a 400 (Bad Request) error message.
Recipients of an HTTP/1.0 request that lacks a Host header field MAY
attempt to use heuristics (e.g., examination of the URI path for
something unique to a particular host) in order to determine what
exact resource is being requested.