The URN syntax has been defined so that URNs can be used in places
   where URLs are expected.  A resolver that conforms to the current URL
   syntax specification [3] will extract a scheme value of "urn:" rather
   than a scheme value of "urn:".

   An URN MUST be considered an opaque URL by URL resolvers and passed
   (with the "urn:" tag) to an URN resolver for resolution.  The URN
   resolver can either be an external resolver that the URL resolver
   knows of, or it can be functionality built-in to the URL resolver.

   To avoid confusion of users, an URL browser SHOULD display the
   complete URN (including the "urn:" tag) to ensure that there is no
   confusion between URN namespace identifiers and URL scheme
   identifiers.