Also, something that nobody has the faintest idea on how to resolve, and an attempt to perpetrate jargon murder
on the useful, well defined, well understood concept of URL
The universal resource name, and its dreaded brethren - the URI - Universal resource identifier, are widespread pollutants of the technical discourse. Enshrined in several RFCs, they tend to be defined in terms of what they are not, vaguely termed as being ``like URLs, but persistent and location independent'' etc. Most often, people will say: ``an URL is of course an URI/URN but an URI/URN may not be an URL...''. The same people will meet the logical sequel ``So what would be an example of a URN which is not also a URL?'' with a blank stare.
Fact is, examples of non-URL URNs can be given, as follows:
These examples are also pretty useless, as they are self referential and nobody appears to be able to actually produce a way to resolve the darn thing.
One has only to read the XML mailing lists to realize the perverse uselessness of the URN idea: the question
``How do I resolve the PUBLIC|SYSTEM DTD identifier...''
is one of the most popular (you guessed right, DTD identifiers are defined as URNs). Obviously, the only instance where this question has a definite answer is when the URN is also a URL.
The reasoning behind the definition of URNs (and URIs) appears to be motivated that a URL, while immensely useful, carries a few real world warts (non persistence being one). The typical Computer Science reaction to this situation is, therefore, to invent an abstract concept from which the warts are removed.
Too bad that in this instance usefulness has been also removed - the only surviving properties of the original appear to be syntax and (partially) semantics. This has been craftily achieved by defining the thing in terms of an abstract resolution service that nobody ever bothered to define or implement.
It is amusing to observe that the universally used and understood URLs are now officially banned from technical literature - which must refer to URNs and URIs.