← ELI5 · Nestor G Pestelos Jr

URL · URN · URI

URL says where. URN says what. URI is the umbrella.

Three words for one job: naming things on the internet.

URI is the umbrella.

URI URL where URN what

Every URL and every URN is a URI. URI is just the family name.

What URI means
  • URI stands for Uniform Resource Identifier.
  • It is the general word for anything that names a resource.
  • URL and URN are two kinds of URI, not rivals of it.

URL says where.

URL — where it is lib.example/shelf-2/book-7

A URL is an address. It points at one shelf in one library.

What URL means
  • URL stands for Uniform Resource Locator.
  • It describes how to reach the resource: which server, which path.
  • Move the book, and the old URL points at nothing.

URN says what.

the book urn:isbn:0451450523 same name, any shelf

A URN is a name. It stays with the book, not with the shelf.

What URN means
  • URN stands for Uniform Resource Name.
  • An ISBN is a good example: the same name in every library.
  • A URN is meant to stay the same even if the thing moves or disappears.

One book can have both.

URL — where it is now URN — what it is lib.example/shelf-2 urn:isbn:0451450523 the book

The URL changes when it moves. The URN never does.

Which one to use
  • The same thing can carry a where and a what at once.
  • The URL is the current shelf. The URN is the permanent name.
  • Fetch now? Use the URL. Cite forever? Use the URN.

URLs break. URNs last.

URL: shelf-2 the book old shelf → urn:isbn:0451450523 the book new shelf

Move the book once, and the old address dies. The name survives.

Why this matters
  • This is why web links go dead when pages move.
  • A URL promises where it is now, not forever.
  • A URN promises the name stays the same. Finding it is a separate step.

Sources

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