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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?

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URN does not identify the availability of the resource, merely the type of resource. It leaves the decision of how to handle the resource to the underlying system. When you see URNs that include web-based resources, there really isn't a "standard" for defining them.

They're 2 sides of the coin. URN is used to identify what something is... URL is used to define how you get it.

http://en.wikipedia.org/wiki/Uniform_resource_name

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So how can a URN be a type of URI, as the definition of a URI is species how to access a resource. What's the point of a URN if a URL does the same plus some? –  Jonathan. Jan 3 '12 at 22:31
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URL doesn't identify what the object is. http://example.com/fish is a url... but doesn't tell me what "fish" is. It may be a .jpg... it may be a html document... it may be a mp3... who knows. a URN tells you exactly what it is... i.e. urn:isbn:0451450523 tells you that you're going to get an ISBN number... with which you can look up the book using some other app... but it doesn't tell you specifically where/how to get it. –  TheCompWiz Jan 3 '12 at 22:35
    
Typically though you get the extension in a URL which tells you what it is. Why not just write the ISBN number without the urn:, it's not like you could click on that and be taken to the book? –  Jonathan. Jan 3 '12 at 22:46
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@Jonathan: URNs are meant to be somewhat structured, machine-readable. A simple "0451450523" can be an ISBN-10, a phone number, anything -- computers don't understand human languages enough to infer the meaning from context. On the other hand, urn:isbn:0451450523 is an easily recognizable ISBN. Depending on the program, it could be made clickable -- for example, it could be automatically translated to a link to a library catalog. –  grawity Jan 3 '12 at 23:08
    
@grawity, @TheCompWiz, so it's like a custom protocol (can't remember the exact name), like on phones you have a tel:01234567890, it should be urn:tel:01296720498, but for some reason the urn: has been dropped (which is more logical given that we don't have uri:http://google.com)? –  Jonathan. Jan 3 '12 at 23:15

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