Uniform Resource Identifier (URI)

• a URI is a string of characters used to identify either a:
  • name
  • resource
• a URI identifies a resource either by location, or a name, or both
• a URI has two specializations known as URL and URN:
  • URL - a URI that specifies a location
  • URN - a URI that specifies a name
  • URI - a URI that specifies both a name and location

Uniform Resource Locator (URL) - locators

• a URL is a subset of the URI that specifies where an identified resource is available and the mechanism for retrieving it
• a URL defines how the resource can be obtained
• a URL does not have to be HTTP URL (http://example.com:80), a URL can also be a FTP URL (ftp://example.com:22) or SMB URL (smb://example.com:139)
• more details

Uniform Resource Name (URN) - names

• a URN is a URI that uses the URN scheme, and does not imply availability of the identified resource

URI - URL - URN

• both URNs and URLs are URIs, and a particular URI may be both a name and a locator at the same time
• the URNs are part of a larger Internet information architecture which is composed of URNs, URCs and URLs

Examples

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 (?)

the ISBN number used for books is in fact a URN, it’s an unambiguous identifier for a given book. But a ISBN number is not a URL as it does not define where the book can be found