A Uniform Resource Locator, also known by the acronym URL, is a means of specifying the location and access method of an object on the Internet. It includes a scheme or protocol name that describes how to access the object. It also includes the network location, along with optional query and fragment identifiers. A very common use for a URL is to direct a browser to a website.
The original concept of a Uniform Resource Locator evolved during the early 1990s. Request For Comments (RFC) 1630 was the first URL standard, released by the Internet Engineering Task Force (IETF) in 1994. A name without an associated location or access method became known as a Uniform Resource Name (URN). Combining the two concepts, the term Uniform Resource Identifier (URI) was born. A URI can be a URN identity, a URL address, or both. It may even refer to an object that is not network-based at all.
In technical circles, the Uniform Resource Locator term is rarely used any longer—URI is preferred. URL remains a popular name with the general public and the press, however. The latest URI specification, released in 2009 as RFC 3986, clarifies the URL, URN and URI concepts.
A Uniform Resource Locator consists of two parts. First is the name of a URI scheme, followed by a colon. A scheme defines the protocol or other method used to access the resource. The Internet Assigned Numbers Authority (IANA) manages a lengthy list of registered URI schemes. Hypertext Transport Protocol (HTTP), Secure HTTP (HTTPS) and File Transport Protocol (FTP) are probably the most common. Many unregistered schemes are also used.
The second part of a Uniform Resource Locator is scheme-specific and may include several components. This part of an HTTP URL often begins with www. followed by a domain name. A numeric Internet Protocol (IP) address may be used in place of the domain name. The name can be followed by the path to a specific object. A query string, the name of a fragment heading within the object, or both can also be present.
When typing the URL of a web page into a browser, the scheme and part of the domain name are frequently optional. If left out, "http://" or "http://www." will usually be assumed by the web browser. The path, query or fragment may also contain limited special characters in hexadecimal numeric form. A space—%20—is the most commonly used one. An Internationalized Resource Identifier (IRI) also allows Unicode characters throughout.
More than one unique Uniform Resource Locator may describe a path to the same object. Search engines can use a process called URL normalization to determine whether multiple URLs actually refer to the same thing. Web browsers and crawlers do this as well. A URL may also point to an object which can not be found—it moved or never existed to begin with.
