WORLDWIDE WEB (WWW)
The acronym WWW stands for World Wide Web (WWW). The World Wide Web is officially described as a wide-arc hypermedia information retrieval initiative aiming to give universal access to a large universe of documents.
The term hypermedia has been derived from hypertext. Hypertext is a text that is non-sequential or non-linear in nature. The reader is able to choose a path to delve deeper into the information. Hypertext describes the ability to link related documents together using words and phrases. Hypermedia is a natural extension of hypertext in that the contents of each document include much more than text. They include multimedia (images, sound, video).
WWW provides a consistent means to access a variety of information in a simplified manner to the users on computer networks.
Tim Benners Lee, a research scientist at the European Particle Physics Laboratory (CERN) in Geneva, Switzerland, developed the concept of WWW in 1.989. The earlier WWW systems were designed for the two main purposes of advancement of science and education. But WWW has made a significant impact not only on these two areas, but many other areas such as commerce, politics and also literature.
Some people consider the WWW to be the Internet. This is not correct. WWW is just one of the many technologies that the Internet makes possible.
 Hypertext and Hypermedia
Technically, the web is a distributed hypermedia system that supports interactive access. A hypermedia system provides a straightforward extension of traditional hypertext system. In either system, information is stored as a set of documents. Besides the basic information, a document can contain pointers to other documents in the net. Each pointer is associated with a selectable item that allows a user to select the item and follow the pointer to a related document. The difference between hypertext and hypermedia arises from document content: hypertext documents contain only textual information while hypermedia documents can contain additional representations of information, including digitized photographic images or graphics.
A hypermedia document available on the Web is called a page; the main page for an organization or an individual is known as a homepage. Because page can contain many items, the format must be defined carefully so that a browser can interpret the contents. In particular, browsers must be able to distinguish among arbitrary text, graphics and links to other pages. More important, the author of a page should be able to describe the general document layout (for example, the order in which items are presented).
Each Web page that contains a hypermedia document uses a standard representation known as the Hypertext Markup Language (HTML). The standard allows an author to give general guidelines for display and to specify the contents of the page. Each HTML document is divided into two major parts: a head followed by a body. The head contains details about the document, while the body contains the majority of the information.

The HTML provides a number of commands that can be used to place and format text, pictures and sound on the web pages. Besides, using HTML a word or a phrase or even a picture can be linked to other documents on the Internet.
Web documents are typically written in HTML, which is very easy to use, with any text editor such as Windows or Notepad. These documents are usually named with the suffix html or htm. Thus, HTML documents are standard ASCII files with formatting codes that contain information about layout (text styles, documents titles, paragraphs, lists) and links. After entering the HTML code in a file you can view it using any browser. The browser automatically interprets the HTML code and formats the document accordingly.
  Hypertext Transfer Protocol
The Hypertext Transfer Protocol (HTTP) is a protocol used mainly to access data on the World Wide Web. The protocol transfers data in the form of plain text, hypertext, audio, video and so on. However, it is called the hypertext transfer protocol because its efficiency allows its use in the hypertext environment where there are rapid jumps from one document to another.
HTTP functions like a combination of FTP and SMTP. It is similar to FTP because it transfers files and uses the services of TCP. However, it is much simpler than FTP because- it uses only one TCP connection. There is not a separate control connection; only data are transferred between the client and the server.
HTTP is like SMTP because the data transferred between the client and the server look like SMTP messages. In addition, the format of the messages is controlled by MIME-like headers. However, HTTP differs from SMTP in the way the messages are sent from the client to the server and from the server to the client. Unlike SMTP, the HTTP messages are not designed to be read by humans; they are read and interpreted by the HTTP server and HTTP client (browser). SMTP messages are stored and forwarded but HTTP messages are delivered immediately.
The idea of HTTP is very simple. A client sends a request, which looks like a mail, to the server. The server sends the response, which looks like a mail reply, to the client. The request and response messages carry data in the form of a letter with MIME-like format.
  Universal Resource Locators
The World Wide Web uses Universal Resource Locators (URL) to represent hypermedia links and links to network services within HTML documents. It is possible to represent almost any file or service on the Internet with a URL. A URL looks like: http://www.microsoft.com
The first part of the URL (before the two slashes) specifies the method of access. The second is typically the address of the computer on which the information or service is to be located. Further parts may specify the names of files, the port to connect to, or the text to search for in a database. A URL is always a single line with no embedded spaces.

The Uniform Resource Locator (URL) is a standard for specifying any kind of information on the Internet. The URL defines four things: method, host computer, port and path.

The method is the protocol used to retrieve the document, for example HTTP. The host is the computer where the information is located, although the name of the computer can be an alias. Web pages are usually stored in computers and computers are given alias names that usually begin with the characters "www". This is mandatory, however, as the host can be any name given to the computer that hosts the web pages. The URL optionally can contain the port number of the server. If the port is included, it should be inserted between the host and the path and it should be separated from the host by a colon. Path is the pathname of the file where the information is located. Note that the path can itself contain slashes that in the UNIX operating system, separate the directories from the subdirectories and files.

Following are some examples of URL:

ftp://www.mcafee.com/pub/readme.txt

http://www.prenhall.com/index file://www.oxford.edu/directory/index
12.6.4 How Does the WWW Work?
The WWW system is based on the client/server architecture. A Web client (that is, a browser such as Netscape Navigator, or Microsoft Internet Explorer) sends request for information to any web server. This is a program upon receipt of which the server sends the document requested for (or an error message if needed) to the requesting client.
Typically the client program (that is, the browser) runs on a separate machine from that of the server. The server takes care of all the issues related to document storage, whereas the task of presenting the information to the user is left to the client program.
The following example illustrates the communication between a web server and the browser (that is, the client):
(1) User connects to the web site www.timesofindia.com using the Netscape Navigator browser. The web server that hosts the www.timesofindia.com site is located and sends the text, graphics and sound associated with the home page to the client (the browser). The browser then displays the home page.

(2) User clicks on the cartoon hyperlink on the Times of India home page. The web client uses the address associated with that hyperlink to connect to the web server at a specified network address (maybe at the same site or on a different server) and asks for the document associated with cartoon hyperlink.

(3) The server responds by sending the text and the graphics associated with the called document to the client. The client then presents the received information on the screen.
The protocol that web clients and servers use to communicate with each other is called the Hypertext Transfer Protocol (HTTP). All web clients and servers must use HTTP in order to send/receive hypermedia documents. For this reason, web servers are often called HTTP servers.

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