URI, URL and URN

• Uniform Resource Identifier—A URI is a string that identifies any resource.
Identifying the resource may not necessarily mean that we can retrieve it. URI is
a superset of URL and URN.
• Uniform Resource Locator—URIs that specify common Internet protocols such
as HTTP, FTP, and mailto are also called URLs. URL is an informal term and is
not used in technical specifications.
• Uniform Resource Name—A URN is an identifier that uniquely identifies a
resource but does not specify how to access the resource. URNs are standardized
by official institutions to maintain the uniqueness of a resource.


Here are some examples:
• files/sales/report.html is a URI, because it identifies some resource.
However, it is not a URL because it does not specify how to retrieve the resource.
It is not a URN either, because it does not identify the resource uniquely.
• http://www.manning.com/files/sales/report.html is a URL because
it also specifies how to retrieve the resource.
• ISBN:1-930110-59-6 is a URN because it uniquely identifies this book,
but it is not a URL because it does not indicate how to retrieve the book.