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Identity and significance

Locator and name

The distinction within the URI family is between locating and naming. A URL says where a resource is and how to reach it; a URN gives a resource a persistent name independent of where it lives. That is the old split between identity and location: a URN like urn:isbn:… identifies a book without committing to any copy of it, while a URL identifies a particular retrievable representation. URI is the umbrella that holds both — identification first, retrieval optional.

Persistence

Because a URL ties identity to a location, it can break when the location changes (“link rot”); the URN idea, and conventions for persistent identifiers, are responses to the wish for names that outlive locations. The tension between a convenient locator and a durable name runs through the whole topic.

Why it mattered

Uniform identification is what let the Web be a web: any resource could link to any other through one addressing scheme, with no central registry of what may be linked. The same idea underpins REST — where a URI names a resource and operations act on it (see Roy Fielding) — and linked data, where URIs name not just documents but things and relationships. The reach goes well beyond the browser: URIs identify message topics, package coordinates, configuration keys, and resources in countless systems — wherever a uniform way to name and refer to things is wanted.

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