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Uniform resource Identfiers

Objects need identifiers. Concepts such as relationships also need identifiers. These need to label things uniquely. This role is played by URIs (Uniform resource Identfiers)


First there were URLs...

Hyperlinks formed a critical part of the World Wide Web from the start. They gave a simple mechanism for pointing from one Web page to another. They were known as URLs (Uniform Resource Locators). However, they were made up of several parts

A typical URL, e.g. for this page is

where http is the scheme, jan.newmarch.name is the addres of my server and SemanticWebForLibraries/URIs/index.html is the name of this resource on my server.

Then there were URNs

But nobody ever used them, so (almost) forget about them. Except they hang around in some places. They begin with urn

And now there are URIs

URLs in a crude sense label where things are. (Crude, because you don't really know where e.g. Microsoft servers are.). But if you wanted to label everything, then, well, not everything has a location. For example, ISBN numbers aren't found at a particular place. Even worse, relationships aren't found anywhere: if "father-of" referred to a particular place then there wold be many paternity suits...

URIs (Uniform Resource Identifiers) are the Web solution to naming things. The basic structure is fairly vague:

scheme : something
The reason for the vagueness is to accomodate as many schemes as possible. For example, http schemes are very precise for labelling where (or which parts of) a resource are located. Other schemes include the ISBN scheme, a Z39.50 Retrieval Scheme, a Z39.50 Session scheme, and many hundreds of others.

Who decides what?

The format of ISBN numbers was established as an ISO standard. How does this and other schemes relate to the Web? Well, it's a complex story!

At the end of this chase, you would come up with the following for ISBN numbers:

URN:ISBN:<:ISBN number>
In a similar way you can find URI encoding schemes for other labelling schemes in existence.


Objects, ideas, relationships need to be labeled in some way. URIs are the Web way of labelling things. While a significant subset is URLs, other shemes such as ISBN can be brought into the URI world. As we shall see later, URIs play a fundamental role in all aspects of the Semantic Web.

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