URIs come in two flavors: URNs and URLs.  URNs and URLs are both URIs, and a URI can be a URN and a URL simultaneously.

• If a URI identifies a resource regardless of the location of that resource, then the URI is a URN (Uniform Resource Name)
• If a URI identifies a (web) location at which a bound resource may be found, then the URI is a URL (Uniform Resource Locator)

Though URIs (including URNs and URLs) must follow the appropriate IETF syntax to be considered URIs, most people are familiar with a number of URI analogs.

For example: Social Security numbers and car license plate numbers function as URNs, in that they use consistent syntax and identify resources without specifying that resource's location. Each individual Social Security number uses consistent syntax (NNN-NN-NNNN) and provides an identifier for a human resource (an individual person). Similarly, a car license plate number uses a combination of 6-7 letters and numbers and identifies a specific vehicular resources (a car).  In both cases, the identifier does not indicate the location of the resource: a Social Security number does not indicate the location at which a specific person may be found, nor does a license plate number indicate the location at which an individual car may be found.

Telephone numbers and street addresses are analogous to URLs, in that they follow a consistent syntax and identify a location at which a resource may be found. For example, street addresses use consistent syntax (NNNN [Street Name] [City] [State] [Zip Code] in the United States) and identify the location at which a resource (a shopping center, business, or office) may be found. Telephone numbers also function as URLs: they follow consistent syntax (NNN-NNN-NNNN) and identify a location (on the switchboard) at which a human resource (a person) may be found.  In both cases, resources are associated with an identified location by means of a binding: a business resource is bound to a street address in the same way that a human resource is bound to a telephone number.  If a business moves to another address or a person is no longer associated with a given phone number, then the binding is invalid.

All web addresses entered into your web browser are URLs. The web site http://www.google.com is a URL that identifies the location at which the Google search engine (a computing resource) may be found.

As with URIs, IETF-compliant syntax is available for both URNs and URLs.

Continue the tutorial by clicking the links below.