URL Relative Syntax and Base URLs
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The Uniform Resource Locator syntax described in the first topic of this section is sometimes said to specify an absolute URL. This is because the information in the URL is sufficient to completely identify the resource. Absolute URLs thus have the property of being context-independent, meaning that one can access and retrieve the resource using the URL without any additional information required.
Since the entire point of a URL is to provide the information needed to locate and access a resource, it makes sense that we would want them to be absolute in definition most of the time. The problem with absolute URLs is that they can be long and cumbersome. There are cases where many different resources need to be identified that have a relationship to each other; the URLs for these resources often have many common elements. Using absolute URLs in such situations leads to a lot of excess and redundant verbiage.
In my overview of URIs I gave a real world analogy to a URL in the form of a description of an access method and location for a person retrieving a book: 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.
What if I also wanted the same person to get a second book located in the same house on the ground floor after getting the first one? Should I start by saying again take the train to Albuquerque, then Bus #11 to 41 Albert Street, a red brick house owned by Joanne Johnson? Why bother, when they are already there at that house? No, I would give a second instruction in relative terms: go back downstairs, and also get the blue book on the wood table. This instruction only makes sense in the context of the original one.
The same need arises in URLs. Consider a Web page located at http://www.longdomainnamesareirritating.com/index.htm that has 37 embedded graphic images in it. The poor guy stuck with maintaining this site doesn't want to have to put http://www.longdomainnamesareirritating.com/ in front of the URL of every image.
Similarly, if we have just taken a directory listing at ftp://ftp.somesitesomewhere.org/very/deep/directory/structures/also/stink/ and we want to explore the parent directory, we would like to just say go up one level without having to say ftp://ftp.somesitesomewhere.org/very/deep/directory/structures/also/.
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