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
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:
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:John.Doe@example.com 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)