Uniform Resource Locator is an URI which also specifies where the identified resource is available and the protocol for retrieving it. In popular usage and many technical documents, it is often confused as a synonym for Uniform Resource Identifier (URI), which are not required to specify how to locate the resource.
URL syntax in brief
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 a http://example.org/
by performing an [
] request to the host example.org
, at the default HTTP port (see Port 80
). Dereferencing the URL mailto:email@example.com
will usually start an e-mail
composer with the address firstname.lastname@example.org
in the To
example.com is a domain name; an IP address or other network address might be used instead. In addition, URLs that specify
 as a scheme (such as https://example.com/) normally denote a secure web site.
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:
- http://en.wikipedia.org/ and HTTP://EN.WIKIPEDIA.ORG/ will both open same page.
- http://en.wikipedia.org/wiki/Url is correct, but http://en.wikipedia.org/WIKI/URL/ will result in an  error page.
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’).”
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.
- RFC 3986 Uniform Resource Identifier (URI): Generic Syntax