Do you know what does URI, URL and URN means? I bet you will say that I know about URL but what about URI and URN! Here is a good description on Wikipedia and that is enough to understand these three:
URI tells a unique path to a resource on the Internet. This image will give you some understanding of the relationship of URI, URL and URN.
Now, wikipedia has a very good explanation for this and I cannot give you better explanation:
Computer scientists may classify a URI as a locator (URL), or a name (URN), or both. A Uniform Resource Name (URN) functions like a person's name, while a Uniform Resource Locator (URL) resembles that person's street address. In other words: the URN defines an item's identity, while the URL provides a method for finding it.
The ISBN system for uniquely identifying books provides a typical example of the use of URNs. ISBN 0486275574 (urn:isbn:0-486-27557-4) cites unambiguously a specific edition of Shakespeare's play Romeo and Juliet. In order to gain access to this object and read the book, one would need its location: a URL address. A typical URL for this book on a Unix-like operating system would be a file path such as file:///home/username/RomeoAndJuliet.pdf, identifying the electronic book saved in a file on a local hard disk. So URNs and URLs have complementary purposes.
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