In practice, it is assumed that most URNs will refer to on-line resources, and that a mechanism will exist which takes a URN and returns a list of URLs[20]. Another, intriguing, possibility is that URNs may be linked to other URNs rather than directly to URLs. This raises the prospect of their being used to establish a semantic network of pointers to resources - a true virtual library!
Most World-Wide Web software packages need only minor modifications to implement support for URNs via the proxy mechanism[21]. This is a feature which allows requests for particular types of URL such as gopher and http to be sent to a server rather than carried out by the browser itself. The proxy feature is widely used to implement caching, and provide access to Internet information services over network security firewalls [22].
There have been a number of URN resolution proposals. The rest of this section discusses those proposals which were under investigation when this report was written.