The problem is that, by definition, a location cannot serve as an identifier. When the resource is moved, the connection is severed. For the most part, URLs have been highly successful tools. They were designed to be a flexible, easily extensible and easily implemented way to reference materials on the Internet. URLs were never, however, intended to be a way to manage specific content for the long term.
URNs are persistent identifiers for Internet resources.
They will eventually be the Internet standard for identifying and finding electronic resources. The problem that we are faced with today is that standard browsers do not yet understand URNs and how to convert them to URLs. A URN must be assignable to any resource available on the Internet. Identifiers have been extremely important standardization tools among publishers, booksellers, and libraries for many years. There are a number of unique identifier services that have been developed for electronic resources.