URI vs. URL vs. URN

[Stanimir Stamenkov]

"Re: aligning input buttons", Mon, 06 Jun 2011 19:37:39 +0300,
/Jukka K. Korpela/:

Can you guess what it might be? I'll give you a hint: Originally, it
was called Universal Resource Identifier, later renamed to Uniform
Resource Locator.

URIs are generalization of URLs and URNs. I'm not sure if one or
the other (URI or URL) emerged first, if at different times. From
my experience URL is really what has been used first (as a term).

... Existing
URI schemes which correspond to the (still mutating) concept of IETF
URLs are listed here...

which kinda suggests to me URLs came first. Anyway, it appears URIs
and URLs have distinct meaning from the very beginning. Here's some
general info from Wikipedia
<http://en.wikipedia.org/wiki/Uniform_Resource_Identifier>:

| *Relationship to URL and URN*
|
| Diagram of URI scheme categories. Schemes in the URL (locator)
| and URN (name) categories form subsets of URI, and also
| (generally) disjoint sets. Technically URL and URN function as
| resource IDs; however, one cannot exactly categorize many schemes
| as one or the other: we can treat all URIs as names, and some
| schemes embody aspects of both categories.
|
| One can classify URIs as locators (URLs), or as names (URNs), or
| as both. A Uniform Resource Name (URN) functions like a person's
| name, while a Uniform Resource Locator (URL) resembles that
| person's street address. In other words: the URN defines an
| item's identity, while the URL provides a method for finding it.
|
| [...]
|
| *Technical view*
|
| A URL is a URI that, in addition to identifying a network-homed
| resource, specifies the means of acting upon or obtaining the
| representation: either through description of the primary access
| mechanism, or through network "location". For example, the URL
| http://www.wikipedia.org/ identifies a resource (Wikipedia's home
| page) and implies that a representation of that resource (such as
| the home page's current HTML code, as encoded characters) is
| obtainable via HTTP from a network host named www.wikipedia.org.
| A Uniform Resource Name (URN) is a URI that identifies a resource
| by name, in a particular namespace. One can use a URN to talk
| about a resource without implying its location or how to access
| it. The resource does not need to necessarily be network homed.
| For example, the URN urn:isbn:0-395-36341-1 is a URI that
| specifies the identifier system, i.e. International Standard Book
| Number (ISBN), as well as the unique reference within that system
| and allows one to talk about a book, but doesn't suggest where
| and how to obtain an actual copy of it.

 

[Jukka K. Korpela]

"Re: aligning input buttons", Mon, 06 Jun 2011 19:37:39 +0300, /Jukka K.
Korpela/:


URIs are generalization of URLs and URNs.

"URI" in its modern usage is basically specs-writers' way of talking
about URLs and pretending that they're referring to something more
general. And the more general stuff exists - in Specs-land. Surely you
can write urn:isbn:9780596101213 and think that you have used a URN, a
cooler form of a URI than URLs are. But so what? Where will it take you,
or your reader to? Googling with the digit string 9780596101213 is much
more successful than trying to find a useful URN resolver.

(In a non-distant future, you will probably be able to run searches
where you specify that you are looking specifically for pages that
mention a string as an ISBN number. But not using URNs. Google, Bing,
and Yahoo are now promoting microdata as specified by their Schema.org.
The approach is much less elegant than the URN concept, but it will be a
success in a year, whereas URNs are, after about two decades of
theoretical development, still just theory, for the most of it.)

I'm not sure if one or the
other (URI or URL) emerged first, if at different times.

Documentation of the early history of the Web is misty and obscure, and
consistence of terminology was not the key virtue of early developers.
From what we can get from the Usenet archives as munged by Google, it
seems that URL was the first one to pop up in public discussions. But
the title of the informational RFC that you cited, RFC 1630, carries the
heading "Universal Resource Identifiers in WWW", with the subheading "A
Unifying Syntax for the Expression of Names and Addresses of Objects on
the Network as used in the World-Wide Web". This may reflect the time
when people said just "address" (or "network address"), then started
trying to formalize the idea (with "names" extension) as "Universal
Resource Identifier". But the generic "identifier" concept was soon
replaced by the idea of a pathname-like address, called "locator" for
some funny reason.

The "U" originally stood for "Universal" but was soon re-interpreted as
"Uniform". In reality, URLs (or URIs) are far from uniform... there's a
large variety of different URL schemes, each with its own internal
structure.

But URLs are still useful, even indispensable.