Учим английский :).
В rfc ничего не сказано про Host:
Wikipedia:
http://en.wikipedia.org/wiki/Shared_web_hosting_serviceDisadvantages:
* They will not work with browsers that do not send the hostname as part
of requests. This is true for older HTTP/1.0 browsers that have not
retrofitted the host field feature from the HTTP/1.1 protocol. (The "Host" header that distinguishes between various
DNS names sharing a single
IP address was optional in HTTP/1.0; it is mandatory in HTTP/1.1, issued in 1999 as
RFC 2616.)
Since nearly every webbrowser that is currently used supports the
HTTP/1.1 protocol and thus also virtual hosting, this is not a real
issue.