A URL (Uniform Resource Locator) is a specific type of URI (Universal Resource Identifier). A URL normally locates an existing resource on the Internet. A URL is used when a Web client makes a request to a server for a resource.
The concepts of the URI and the URL are defined by the Internet Society and IETF (Internet Engineering Task Force) Request for Comments document RFC 2396, Uniform Resource Identifiers (URI): Generic Syntax (http://www.ietf.org/rfc/rfc2396.txt). Briefly, a URI is defined as any character string that identifies a resource. A URL is defined as those URIs that identify a resource by its location or by the means used to access it, rather than by a name or other attribute of the resource.
A URL for HTTP (or HTTPS) is normally made up of three or four components:
- A scheme. The scheme identifies the protocol to be used to access the resource on the Internet. It can be HTTP (without SSL) or HTTPS (with SSL).
- A host. The host name identifies the host that holds the resource. For example, www.example.com. A server provides services in the name of the host, but there is not a one-to-one mapping between hosts and servers. Host names explains more about host names.Host names can also be followed by a port number. Port numbers explains more about these. Well-known port numbers for a service are normally omitted from the URL. Most servers use the well-known port numbers for HTTP and HTTPS , so most HTTP URLs omit the port number.
- A path. The path identifies the specific resource within the host that the Web client wants to access. For example,/software/htp/cics/index.html.
- A query string. If a query string is used, it follows the path component, and provides a string of information that the resource can use for some purpose (for example, as parameters for a search or as data to be processed). The query string is usually a string of name and value pairs, for example,q=bluebird.
The components of the URL are combined and delimited as follows:
scheme://host:port/path?query
- The scheme is followed by a colon and two forward slashes.
- If a port number is specified, that number follows the host name, separated by a colon.
- The path name begins with a single forward slash.
- If a query string is specified, it is preceded by a question mark.
Figure 1. Syntax of an HTTP URL
This is an example of an HTTP URL:http://www.research.ibm.com/software/htp/cics/index.htmlIf a port number was specified, the URL would be:
http://www.research.ibm.com:1030/software/htp/cics/index.html
Other protocols, such as File Transfer Protocol (FTP) or Gopher, also use URLs. The URLs used by these protocols may have a different syntax to the one used for HTTP.
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