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Table Of Contents  The TCP/IP Guide
 9  TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)
      9  TCP/IP Key Applications and Application Protocols
           9  TCP/IP Application Layer Addressing: Uniform Resource Identifiers, Locators and Names (URIs, URLs and URNs)

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Uniform Resource Locators (URLs)
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Uniform Resource Identifiers, Locators and Names (URIs, URLs and URNs): Overview, History, Significance and Standards
(Page 2 of 3)

URI Categories

URIs are in fact a general purpose method for referring to many kinds of TCP/IP resources. They are currently divided into two primary categories based on how they describe a resource:

  • Uniform Resource Locators (URLs): A URL is a uniform resource identifier that refers to a resource through the combination of a protocol or access mechanism and a specific resource location. A URL begins with the name of the protocol to be used for accessing the resource and then contains sufficient information to point to how it can be obtained.

  • Uniform Resource Names (URNs): A URN is a uniform resource identifier that provides a way of uniquely naming a resource without specifying an access protocol or mechanism, and without specifying a particular location.

The difference between a URL and a URN is that the former is much more specific and oriented around how to access a resource, while the latter is more abstract and designed more to identify what the resource is than describe how to get it. Giving someone a URL is like giving them directions to find a book, as follows: “Take the train to Albuquerque, then Bus #11 to 41 Albert Street, a red brick house owned by Joanne Johnson. The book you want is the third from the right on the bottom of the bookshelf on the second floor”. A URN is more like referring to a book using its International Standard Book Number (ISBN); it uniquely identifies the book regardless of where the book may be located, and doesn't tell you how to access it. (In fact, ISBNs are one of the identification systems used with URNs.)

While URLs and URNs are theoretical peers, in practice, URLs are used far more than URNs. In fact, URLs are so dominant that most people have never even heard of URIs or URNs. The reason is that even though the example above suggests that URNs are more “natural” than URLs, URLs are easier to use in practice—they provide the information needed to access a resource, and without being able to access a resource, simply knowing how to identify it is of limited value. URNs are an attractive concept because they identify a resource without tying it to a specific access mechanism or location. However, the implementation of URNs requires some means of tying the permanent identifier of a resource to where it is at any given moment, which is not a simple task. For this reason, URNs and the methods for using them have been in development for a number of years, while URLs have been in active use all that time.

While URLs began with the Web and most URLs are still used with HTTP, they can and do refer to resources that are accessed using many other protocols, such as FTP and Telnet. Again here, the compactness of URIs makes them very powerful; with a URL, we can use one string to tell a program to retrieve a file using FTP. This replaces the complete FTP process of starting an FTP client, establishing a session, logging in and issuing commands.

Key Concept: Some sort of mechanism is needed on any internetwork to allow resources such as files, directories and programs to be identified and accessed. In TCP/IP, Uniform Resource Identifiers (URIs) are used for this sort of “application layer addressing”. The two types of URIs are Uniform Resource Locators (URLs), which specify how to access an object using a combination of an access method and location, and Uniform Resource Names (URNs), which identify an object by name but do not indicate how to access it.



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TCP/IP Application Layer Addressing: Uniform Resource Identifiers, Locators and Names (URIs, URLs and URNs)
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Version 3.0 - Version Date: September 20, 2005

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