<blockquote>Can someone explain to me why a host: tag is required and how my browser knows to use it.
</blockquote>
Isn't it the case that your browser always has to send it if it is sending a HTTP/1.1 request: http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.23
"A client MUST include a Host header field in all HTTP/1.1 request messages . If the requested URI does not include an Internet host name for the service being requested, then the Host header field MUST be given with an empty value. An HTTP/1.1 proxy MUST ensure that any request message it forwards does contain an appropriate Host header field that identifies the service being requested by the proxy. All Internet-based HTTP/1.1 servers MUST respond with a 400 (Bad Request) status code to any HTTP/1.1 request message which lacks a Host header field."
Steve
