URLs (uniform resource locators), like URNs (uniform resource names), are a subset of the general class of uniform resource identifiers. The distinction between URI, URL, URN, and the rest of the sometimes described URx identifiers is often more confusing than useful. We'll confine ourselves to the term URI, meaning all of them.

In general resource is a time varing conceptual mapping to a set of entities or values which are equivalent . For example the resource identified by the URI http://www.w3.org names the concept of the W3C home page which is itself a mapping to set of values returned as hypertext. The value returned (the hypertext) that changes frequently as new infomation displaces old information from the home page. The set of values mapped by a resource are equivalent resource representations and/or resource identifiers (giving further indirection or redirection). Dereferencing a resource identifier yields a representation of the current value of the referenced resource. At some time, t, the set of values that a resource maps to may be empty, which allows a concept to be identified before a realisation of the concept exists (or indeed after it has been retired).

For example, for the W3C technical report collection it is common practice to assign a URI which always references the current version of a given technical report. In addition, each published version of a technical report is assigned a distinct URI that references that specific version. These two URIs reference two different resources or two different concepts: a specific version of a technical report and the current version of that same report. At some point in time dereferencing either URI may yield the same resource representation. However, at some later instant dereferencing the URI that references the current version of the technical report may yield a different set of representations than deferencing the resource referenced by the version specific URI. With the version specific URI there is a commitment, as a matter of W3C policy, that the set of resource representations referenced by that URI will not change over time, whereas, with the URI that references the current version of the report, there is a commitment, as a matter of W3C policy, that the resource representations referenced by that URI will always represent the most up-to-date published version of the report.

The important point to note is that in general a resource is a time varying mapping to a value, and not simply the value returned by deferencing the resource at a particular moment in time.

A further point to note about resources is that their identifiers expose nothing more than their identity (see Opaque below). It is a matter for the authority that assigns an identifier to a resource to say what that resource means and what commiment it makes to sustainin the meaning of that resource.

is clear that "A resource can be anything that has identity". This is not a closed definition. Are there more things that can be regarded as resources than just those with assigned URIs (or URI references)? RDF provides the ability to described resources by their relationship to one another which leads to the notion of existentally qualified resources. For example, there exists a person whose internet mailbox is identified by the URI mailto:timbl@w3.org. This identifies the person of Tim Berners-Lee by reference to the URI of his internet mailbox without it being necessary to assign a URI to identify the concept of the person Tim Berners-Lee. Of course dereferencing such a resource might prove to be an interesting challenge.

It is appealing to ask when two resources are the same resource, or perhaps more particularly when two resource identifiers identify the same resource. These are difficult questions. Two different URI's may identify the same resource, but it is only the authorities that asssign those URIs that can make the commitment to them identifying the same resource.

However, as they say "Cool URIs don't change" . There are strong social expectations that once a URI is assigned to identify a particular resource, then it should continue indefinitely to refer to that same resource or concept. This is of course best practice, but it is a matter of policy and commitment on the part of authorities assigning URIs rather than a constraint imposed by technological means.

We are dealing here with two time dependent mappings. Firstly a time dependent maping between and identifier and a resource and then a time dependent mapping as describe above, between a resource and a set of equivalent values. The binding between an identifier and a resource is not fixed for all time and neither is the mapping between a resource and its current value. At some instant in time two URIs may reference the same resource, and at some later instant they may reference different resources. It is the authority controlling the assignment of a given URI to identify a particular resources that determine what resource a given URI references at a given time. Some URI schemes may give stronger guarantees about the temporal stability of the URI to resource mapping. Some organisations as a matter of policy may give stronger guarantees than those intrinsic to the schemes that they use for assigning URIs.

The assignment of meaning and resources to an identifier comes under the control of some authority (much as a parent assigns a name to a child, although giving it meaning is perhaps a little harder). We can only know that two identifiers reference the same resource because the authorities that assign the identifiers assert (directly or indirectly) that they identify the same resource, and even then, such assertions may not hold for all time.

If a URI contains an sharp character (a # ), the string that follows the # is a fragment identifier. Fragment identifiers are a mechanism for identifying part of a resource. For example, in a URI that retrieves an HTML document (a resource representation), the fragment identifier #foo can be used to reference the element with the ID foo within that document.

This means that in general, it's not possible to determine what a fragment identifier means without retreiving the resource into which it points.

The fragment identifier identifies some sub-part of a resource representation. The syntax and interpretation of a fragment identifier is determined by the MIME media type of a resource representation. This is considered a design flaw .

A URI that consists of only a fragment identifier (i.e, one that begins with a # ) always points into the document that contains the URI, irrespective of the effective base URI.