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What is the difference between URI, URL and URN?

What's the difference between an URI, URL and URN? I have read a lot of sites (even Wikipedia) but I don't understand it.

URI: http://www.foo.com/bar.html
URL: http://www.foo.com/bar.html
URN: bar.html

Is this correct?

Solution

  • A Uniform Resource Identifier (URI) is a string of characters used to identify a name or a resource on the Internet.

    A URI identifies a resource either by location, or a name, or both. A URI has two specializations known as URL and URN.

    A Uniform Resource Locator (URL) is a subset of the Uniform Resource Identifier (URI) that specifies where an identified resource is available and the mechanism for retrieving it. A URL defines how the resource can be obtained. It does not have to be a HTTP URL (http://), a URL can also start with ftp:// or smb://, specifying the protocol that's used to get the resource.

    A Uniform Resource Name (URN) is a Uniform Resource Identifier (URI) that uses the URN scheme, and does not imply availability of the identified resource. Both URNs (names) and URLs (locators) are URIs, and a particular URI may be both a name and a locator at the same time.

    This diagram (source) visualizes the relationship between URI, URN, and URL:

    URI, URN, URL diagram

    The URNs are part of a larger Internet information architecture which is composed of URNs, URCs and URLs.

    bar.html is not a URN. A URN is similar to a person's name, while a URL is like a street address. The URN defines something's identity, while the URL provides a location. Essentially URN vs. URL is "what" vs. "where". A URN has to be of this form <URN> ::= "urn:" <NID> ":" <NSS> where <NID> is the Namespace Identifier, and <NSS> is the Namespace Specific String.

    To put it differently:

    • A URL is a URI that identifies a resource and also provides the means of locating the resource by describing the way to access it
    • A URL is a URI
    • A URI is not necessarily a URL

    I'd say the only thing left to make it 100% clear would be to have an example of an URI that is not an URL. We can use the examples in RFC3986:

    URL: ftp://ftp.is.co.za/rfc/rfc1808.txt
URL: http://www.ietf.org/rfc/rfc2396.txt
URL: ldap://[2001:db8::7]/c=GB?objectClass?one
URL: mailto:[email protected]
URL: news:comp.infosystems.www.servers.unix
URL: telnet://192.0.2.16:80/
URN (not URL): urn:oasis:names:specification:docbook:dtd:xml:4.1.2
URN (not URL): tel:+1-816-555-1212 (disputed, see comments)