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Venn Diagram illustrating
URI-URL-URN relationship

A URI is a "Uniform Resource Indicator" it is a string of text which uniquely identifies a resource. It is a superset of the URL.

URI's consist of regularly identifiable parts, the protocol scheme, host-name, domain-name, a path and occasionally other information. This is best shown by examining an example as shown below.

The parts of a URI

A URI may be a URL or a URN. An ISBN is a common example of a URN, it is a number which uniquely, across the whole world identifies a book. Typical URIs entered into web-browsers such as Firefox are URLs.

This is semantically confusing, technically speaking a URL is a URI that in addition to locating the resource provides a means to interact with it (such as by specifying the HTTP protocol pursuant to certain conventions). The Subtle difference with URNs is that URNs do not imply availability of the resource in questions, simply having the ISBN of a book does not mean that the book is made available in some way. Having the URI/URL of a PDF copy of that book does imply availability.

To put it simply, URNs say "what" something is, what its name is and URL's tell one where to find it, both are encapsulated within the URI addressing system, URNs can be expressed as URIs using the "URN" scheme designator.

The W3C and other technical bodies strongly discourage the use of the term URL in most cases for various reasons, and indeed as all URLs are a form of URI the need in technical contexts to distinguish between the two rarely arises. Accordingly VNS encourages the use of the term URI in most cases.

Categories: Web, Internet

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