With an URL, it is fairly obvious how it identifies a resource. it simply says where it is. Both a URL and URN are type of URI, so they both are meant to specify how to access a resource.
However I don't understand how a URN specifies how to access a resource? It does not specify where the resource is, just the name of it? How can you identify and access anything on the Internet (or network) using just the name of the resource, whereas a URL specifies on what machine/sub network the resource is, where on the machine it is, and the name (and type) of it?