Uniform Resource Locator (URL)

In computing, a uniform resource locator (URL) is a specific character string that constitutes a reference to an Internet resource.

A URL is technically a type of uniform resource identifier (URI) but in many technical documents and verbal discussions URL is often used as a synonym for URI.
 

History

 
In computing, a uniform resource locator (URL) is a specific character string that constitutes a reference to an Internet resource. A URL is technically a type of uniform resource identifier (URI) but in many technical documents and verbal discussions URL is often used as a synonym for URI.
 

Syntax

 
Every URL consists of some of the following: the scheme name (commonly called protocol), followed by a colon, two slashes, then, depending on scheme, a server name (exp. ftp., www., smtp., etc) followed by a dot (.) then a domain name (alternatively, IP address), a port number, the path of the resource to be fetched or the program to be run, then, for programs such as Common Gateway Interface (CGI) scripts, a query string, and an optional fragment identifier.

The syntax is:-

scheme://domain:port/path?query_string#fragment_id

• The scheme name defines the namespace, purpose, and the syntax of the remaining part of the URL. Software will try to process a URL according to its scheme and context. For example, a web browser will usually dereference the URL http://example.org:80 by performing an HTTP request to the host at example.org, using port number 80. The URL mailto:bob@example.com may start an e-mail composer with the address bob@example.com in the To field.

Other examples of scheme names include https:, gopher:, wais:, ftp:. URLs with https as a scheme (such as https://example.com/) require that requests and responses will be made over a secure connection to the website. Some schemes that require authentication allow a username, and perhaps a password too, to be embedded in the URL, for example ftp://asmith@ftp.example.org. Passwords embedded in this way are not conducive to secure working, but the full possible syntax is

scheme://username:password@domain:port/path?query_string#fragment_id

• The domain name or IP address gives the destination location for the URL. The domain google.com, or its IP address 72.14.207.99, is the address of Google's website.
• The domain name portion of a URL is not case sensitive since DNS ignores case: http://en.example.org/ and HTTP://EN.EXAMPLE.ORG/ both open the same page.
• The port number is optional; if omitted, the default for the scheme is used. For example, http://vnc.example.com:5800 connects to port 5800 of vnc.example.com, which may be appropriate for a VNC remote control session. If the port number is omitted for an http: URL, the browser will connect on port 80, the default HTTP port. The default port for an https: request is 443.
• The path is used to specify and perhaps find the resource requested. It is case-sensitive, though it may be treated as case-insensitive by some servers, especially those based on Microsoft Windows. If the server is case sensitive and http://en.example.org/wiki/URL is correct, http://en.example.org/WIKI/URL or http://en.example.org/wiki/url will display an HTTP 404 error page, unless these URLs point to valid resources themselves.
• The query string contains data to be passed to software running on the server. It may contain name/value pairs separated by ampersands, for example ?first_name=John&last_name=Doe.
• The fragment identifier, if present, specifies a part or a position within the overall resource or document. When used with HTTP, it usually specifies a section or location within the page, and the browser may scroll to display that part of the page.

Absolute and relative URLs

 
According to RFC 1738, which defined URLs in 1994, when resources contain references to other resources, they can use relative links to define the location of the second resource as if to say, "in the same place as this one except with the following relative path". It went on to say that such relative URLs are dependent on the original URL containing a hierarchical structure against which the relative link is based, and that the ftp, http, and file URL schemes are examples of some that can be considered hierarchical, with the components of the hierarchy being separated by "/".
 

URLs as locators

 
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)".
 

Internet hostnames

 
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.example.org consists of a local hostname (en) and the domain name example.org. The 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. For example, both "en.example.org" and "example.org" can be hostnames if they both have IP addresses assigned to them. The domain name "xyz.example.org" may not be a hostname if it does not have an IP address, but "aa.xyz.example.org" may still be a hostname. All hostnames are domain names, but not all domain names are hostnames.

References

1. RFC 3305 "URI Partitioning: There is some confusion in the web community over the partitioning of URI space, specifically, the relationship among the concepts of URL, URN, and URI. The confusion owes to the incompatibility between two different views of URI partitioning, which we call the 'classical' and 'contemporary' views."
2. RFC 1738 Uniform Resource Locators (URL). This RFC is now obsolete. It has been superseded by a newer RFC (see the RFC Index)
3. "Living Documents BoF Minutes". W3.org. Retrieved 2011-12-26.
4. "URL Specification". Retrieved 2011-12-26.
5.  Berners-Lee, Tim. "Frequently asked questions by the press". Retrieved 2010-02-03.
6. "Technology | Berners-Lee 'sorry' for slashes". BBC News. 2009-10-14. Retrieved 2010-02-14.
7. RFC 1738
8. "PHP parse_url() Function". Retrieved 2009-03-12.
9. "URL Syntax". Pangea.stanford.edu. 2004-07-20. Retrieved 2011-12-26.
10. Berners-Lee, Tim; et al (December 1994). "Uniform Resource Locators (URL)". IETF. Retrieved 20 November 2010. "Hierarchical schemes and relative links. In some cases, URLs are used to locate resources that contain pointers to other resources. In some cases, those pointers are represented as relative links where the expression of the location of the second resource is in terms of "in the same place as this one except with the following relative path". Relative links are not described in this document. However, the use of relative links depends on the original URL containing a hierarchical structure against which the relative link is based. Some URL schemes (such as the ftp, http, and file schemes) contain names that can be considered hierarchical; the components of the hierarchy are separated by "/"."
11. Tim Berners-Lee, Roy T. Fielding, Larry Masinter. (January 2005). “Uniform Resource Identifier (URI): Generic Syntax”. Internet Society. RFC 3986; STD 66.
12. By describing its primary access mechanism