Today I came across an informate text/plain document, namely RFC 3305. The longer title for the document is Report from the Joint W3C/IETF URI Planning Interest Group: Uniform Resource Identifiers (URIs), URLs, and Uniform Resource Names (URNs): Clarifications and Recommendations.
The document talks for a bit on the nature of URLs and how they relate to URIs and URNs. What their original meaning was, and what they are today. If you want to know something about it I guess you should give it a read.
In short, a URL is an identifier for a network retrievable resource. URIs (now IRIs) are a superset of URLs and define a general syntax on which new schemes, like the urn scheme, can be based. (Older schemes, like http are covered as well, obviously.) IANA has a list of registered URI schemes.
Where URLs (although we should probably avoid that term in favor of something more complicated, like network retrievable resource identifier or so) point to a location, URNs point to an instance of a document. For example, to refer to an IETF document I could use the ietf URN namespace. (List of official URN namespaces.) For example, if I want to point to the specification that defines XML Media Types I could use urn:ietf:rfc:3023. To point to the specification that defined the ietf namespace I would use urn:ietf:rfc:2648. Using a link however, is easier for the both of us.
(At the bottom of January 2005 Travelog from Ian you can find an example of a URN using the isbn scheme, which is defined in urn:ietf:rfc:3187 as you can find here. O wait, I already mentioned that.
How are we supposed to use URNs when current browsers don't include support for them? For a standard that has been around for such a long time, why isn't there much support for it.? I think it would be nice to see URN support added into Firefox and other browsers.
What kind of support are you hoping for? As I see it, they are mainly useful for being identifiers. Just like tag URIs.
However, as much as how I like to see things done, there is a bug for URN support in Mozilla.
For ISDN numbers, it’s cool. You can make hyperlinks to information not available on the internet. Of course it’s one thing to have such links - it is another for the browser to do something useful with it (like searching for it on Amazon).
~Grauw
There's also a URN plugin for Firefox, although I haven't had time to figure out which namespaces it supports.
I've just used the URN firefox plugin and it seemed to work for isbn, issn and ietf URNs in my article on URNs and citations.
FWIW, you can find slightly hypertextified versions as RFC.net – in this case, http://rfc.net/rfc3305.html. They’re only tagged up ever so lightly, but I find even those few presentational cues tremendously help readability.