URI vs. URL

I've wanted a good operational definition for when you should use URI or URL and so here's my attempt:

Summary:

URI refers to a resource.  e.g. urn:isbn:0-395-36341-1 for a book by ISBN

A URL is a type of a URI that provides additional information that URIs don't necessarily provide (but they can):
* URLs tell WHERE you can obtain a particular representation of a resource -- hence the "Locator" in the name.
** e.g., you use HTTP or FTP to access this server at this address and this specific resource (GIF file, PHP page, etc.)

A resource with a given URI could have multiple different URLs.  The same ISBN URI above can be found at Amazon or many other online URLs, for example.

Ajaxian » URI vs. URL: What’s the difference?

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