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The acronym URL stands for the phrase Uniform Resource Locator, and it refers to the representation people use to locate sources and destinations on the World Wide Web. The content of the URL is called a representation because it is often made more readable to humans than what they would see without it, which would be the protocol, usually http:// for HyperText Transfer Protocol, and the Internet Protocol Address (IP Address) that it stands for. An IP address is made up of four three-digit numeric segments with a value between 0 and 255, and separated by dots. The URL substitutes the domain name or its sub-domain for the IP address. An absolute URL is one type of URL; the other type is a relative URL, and the choice to use a relative or absolute URL may most commonly arise in an HREF (Hypertext REFerence), the code used to link to another page or another item.
Relative means “in relation to,” and a relative URL tells a URL location in terms of the current location. Suppose a webmaster is setting up a website and links an image called "picture.jpg" on the home page in an HREF, using a relative URL that specifies a folder at the same level within the same hierarchy and labeled "Images.” Since the folder with the image is at the same level, the relative address for finding the image is /Images/picture.jpg. That’s all that’s needed because it’s enough information to get there from the current here. Since the relationship of “here” and “there” can change, an absolute URL is a better choice.
An absolute URL gives complete location information. It begins with http:// and continues, including every detail. This means that if the webmaster were to change the location of the original file, an absolute URL specification would find the image, while the relative URL specification would not.
There are several other important characteristics that make absolute URLs valuable. First, an absolute URL is a unique location. Whereas there might be many files called picture.jpg in files called Images across the Internet, the absolute URL refers to only one possible location and one possible file. Additionally, links that go to a separate site, i.e., a location with a different domain name, must be designated by an absolute URL in order to be located.