HTTP/1.0 does not bring host, and HTTP/1.1 adds host head.
Host can be a domain name, or IP, or port number.
Host can be customized by programs. Some programs can define false host in order to prevent operators or bypass firewalls.
Host in HTTP/1.1 can be null value but not without. If no host head is taken, 400 Bad request will be returned.
The HTTP response header does not contain the host field.
Some sites do not check host and can pass arbitrary values.
testrest.cloudapp.net:433