URL is an acronym for Uniform Resource Locator. URL is the technical term for what is often called a "web address". It is the address by which computers and documents on the Internet can be located. The URL was invented by Tim Berners-Lee as one of his many contributions to the World Wide Web. The URL is a short text string that contains the name of a computer on the Internet, a protocol for communicating with that computer, a path and filename of a document on that computer, and sometimes additional information as well. The inclusion of all this information in a single string is allows for seamless interaction between computers on the Internet and rapid exchange of information. The URL was designed to be extensible in the sense that as new types of documents are added to the Internet, the URL would be able to adapt and uniquely identify each document with regard to each new document format's needs.
A full URL consists of a service name, followed by a colon and a hostname, optionally followed by a colon and a port number, followed by a path which ends in a filename or directory. An example of a full URL looks like this: "service:hostname.com:80/directory/file.html". Most URLs that are handled by humans are the addresses of documents on the World Wide Web. These URLs usually specify the HTTP protocol and have the port number eliminated. Additionally, the hostname of a WWW address begins with two forward slashes. An example of such a URL looks like this: "http://hostname.com/". Note that in this example the path specified is a single forward slash, and no file name was specified. This tells the webserver that you are requesting the default file in the topmost public directory of the server. If the webserver has no default file configured then it may return either a list of the files in the directory or an error. A common user error is to specify a URL with no path. Although most webservers are configured to handle this error by automatically assuming that the user wants the default document in the topmost directory, this causes unnecessary server load and may lead to the wrong document being served.
Although a URL specifies a specific document's location on the Internet, electronic documents can be easily copied and therefore may have more than one location. Hence, the concepts of URN (Uniform Resource Name) and URI (Uniform Resource Identifier) have been developed. URNs specify the name of a particular resource, regardless of it's location. URIs are simply complete URLs in which the filename is a URN. For example, two webservers may both have a copy of the book 1984. The book may have any filename, but the International Standard Book Number for 1984 will always be "ISBN 0-452-28423-6". Therefore, the URN of the book 1984 would logically be "isbn.0-452-28423-6". A URL of the type "http://server1.com/isbn.0-452-28423-6" would therefore be a URI for the book 1984. Another URI for the same book could possibly be "http://server2.net/books/isbn.0-452-28423-6". Both URIs would ideally point to identical documents. The difference between a URL and a URI is often arbitrary, as most documents on the World Wide Web are not named by any standards organization.
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