Page 57 - Building Digital Libraries

CHAPTER 3

FIGURE 3.1
Differences between URI, URL, and URN

URNs and URIs do not need to look like computer addresses—they
only need to be unique strings. For example, ISBNs and ISSNs are URNs
and therefore are also URIs by definition. In figure 3.1, all the forms of the
identifier are URIs. The string 10.6083/M4NZ86RR is a URI, but it is not a
URN because it is not explicitly identified as a DOI—that same string could
be used to identify resources that is not a DOI (Digital Object Identifier).
The string doi:10.6083/M4NZ86RR is a URN because it unambiguously
identifies the resource as a DOI and can be resolved using a mechanism
of choice. The string http://doi.org/10.6083/M4NZ86RR is a URL because
that takes someone directly to the resource on a network. Notice that while
a URL uniquely identifies a resource on the network, these are subject to
change because the object may be moved.
Handles, DOIs, and PURLs (Persistent Uniform Resource Locators)
are all URIs that use an unchanging identifier to direct users to a resource.
Identifying an object and directing a user to that object are separate func-
tions, but it is desirable to use the identifier to do both. A PURL is just an
address on the Web that redirects the browser to another address. PURLs
look like regular URLs. Consider the following entry for
http://purl.oclc.org/digitallibraries/book

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