HTTP/1.1 origin servers SHOULD be aware that 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. (But see section 19.5.1 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 hostnames) MUST use the following rules for determining the requested resource on an HTTP/1.1 request:
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.