- If request.url is 'http://d.a.b.c', request.subdomain returns 'd.a'
- If request.url is 'http://a.b.c', request.subdomain returns 'a'
- If request.url is 'http://b.c', request.subdomain returns empty string.
- If request.url is 'http://c', request.subdomain returns empty string.
Whether port number is used does not affect return values.. of course.