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In computing, a Uniform Resource Locator (URL) is a type of Uniform Resource Identifier (URI) that specifies where an identified resource is available and the mechanism for retrieving it.^[1] In popular usage and in many technical documents and verbal discussions it is often, imprecisely and confusingly, used as a synonym for uniform resource identifier. The confusion in usage stems from historically different interpretations of the semantics of the terms involved.^[2] In popular language a URL is also referred to as a Web address.

[edit] Syntax

Main article: URI scheme#Generic syntax

Every URL begins with the scheme name that defines its namespace, purpose, and the syntax of the remaining part of the URL. Most Web-enabled programs will try to dereference a URL according to the semantics of its scheme and a context-vbn. For example, a Web browser will usually dereference the URL http://example.org/ by performing an HTTP request to the host example.org, at the default HTTP port (port 80). Dereferencing the URL mailto:bob@example.com will usually start an e-mail composer with the address bob@example.com in the To field.

example.com is a domain name; an IP address or other network address might be used instead. In addition, URLs that specify https as a scheme (such as https://example.com/) normally denote a secure website.

The hostname portion of a URL, if present, is case insensitive (since the DNS is specified to ignore case); other parts are not required to be, but may be treated as case insensitive by some clients and servers, especially those that are based on Microsoft Windows. For example:

1. http://en.wikipedia.org/ and HTTP://EN.WIKIPEDIA.ORG/ will both open same page.
2. http://en.wikipedia.org/wiki/URL is correct, but http://en.wikipedia.org/WIKI/URL/ will result in an HTTP 404 error page.

[edit] URLs as locators

In its current strict technical meaning, a URL is a URI that, “in addition to identifying a resource, [provides] a means of locating the resource by describing its primary access mechanism (e.g., its network ‘location’).”^[3]

[edit] Internet hostnames

Main article: Hostname

On the Internet, a hostname is a domain name assigned to a host computer. This is usually a combination of the host's local name with its parent domain's name. For example, "en.wikipedia.org" consists of a local hostname ("en") and the domain name "wikipedia.org". This kind of hostname is translated into an IP address via the local hosts file, or the Domain Name System (DNS) resolver. It is possible for a single host computer to have several hostnames; but generally the operating system of the host prefers to have one hostname that the host uses for itself.

Any domain name can also be a hostname, as long as the restrictions mentioned below are followed. So, for example, both "en.wikimedia.org" and "wikimedia.org" are hostnames because they both have IP addresses assigned to them. The domain name "pmtpa.wikimedia.org" is not a hostname since it does not have an IP address, but "rr.pmtpa.wikimedia.org" is a hostname. All hostnames are domain names, but not all domain names are hostnames.

[edit] See also

• CURIE (Compact URI)
• Extensible Resource Identifier (XRI)
• Internationalized Resource Identifier (IRI)
• Uniform Resource Identifier (URI)
• URI scheme
• URL normalization

[edit] References

[edit] External links

• RFC 3986 Uniform Resource Identifier (URI): Generic Syntax [Text] [HTML]