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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)

TCP/IP Application Layer Addressing: Uniform Resource Identifiers, Locators and Names (URIs, URLs and URNs) 1 2 3 Uniform Resource Locators (URLs)

If you've been “working your way up” the OSI Reference Model layers in reading this Guide, you might well have expected that you'd be done with addressing by this point. After all, we have already discussed MAC addresses at layer two and IP addresses at layer three, and mechanisms for converting between them. We even have ports and sockets that provide transport layer addressing capabilities to let each device run multiple software applications. Given all this, the idea of “application layer addressing” may seem a bit strange, and I am aware that using the term to refer to the subject of this section may be a bit unorthodox.

The concept of addressing for applications isn't really as odd as it might seem at first, however. It's true that with an IP address and a port number we can theoretically access any resource on a TCP/IP internet—the problem is finding it. Application layer addressing is not something that is required by the computer software; it is something that makes it easier for humans to identify and locate resources.

This is in fact very much the same rationale that is used to justify the creation of name systems, such as the Domain Name System (DNS). DNS is a form of high-level addressing that allows names to be used instead of IP addresses. It too is important not so much for computers but rather for people, who understand what “www.intel.com” means much more than “198.175.96.33”.

The idea behind a comprehensive application layer addressing scheme is to extend to the next level what DNS has already accomplished. DNS names provide essential high-level abstract addressing, but only of whole devices (whether real or virtual). These names can be used as the basis for a more complete labeling scheme that points not just to a site or device, but to a specific file, object or other resource. In TCP/IP, these labels are called Uniform Resource Identifiers (URIs).

URIs were one of the key technologies developed as part of the World Wide Web (WWW), and are still most often associated with WWW and the protocol that implements it, HTTP. You have likely used URIs thousands of times in the past; whenever you have entered something like “http://www.myfavoritewebsite.com” into a Web browser, you were using a URI. (URI? Isn't that a URL? I'm getting there, I promise.)

The reason why URIs are so important to the Web is that they combine into one string all of the information necessary to refer to a resource. This compactness of expression is essential to the entire concept of hypertext resource linking. If we want to be able to have an object in one document point to another, we need to have a simple way of describing that object without requiring a whole set of instructions. URIs allow us to do exactly that.

In fact, URIs are so associated with the Web that they are usually described as being part of Web technology specifically. They are not, however, unique to the Web, which is why this section is separate from that discussing WWW/HTTP.

TCP/IP Application Layer Addressing: Uniform Resource Identifiers, Locators and Names (URIs, URLs and URNs) 1 2 3 Uniform Resource Locators (URLs)