Computer Education

World Wide Web
A WEB OF COMPUTERS
HOW THE WEB WORKS
WHO USES THE WEB
HISTORY
FUTURE TRENDS

PROGRAMMING LANGUAGES
Machine Language
Assembly Language
High-Level Languages
FLOW-MATIC
FORTRAN
BASIC

HOW COMPUTERS WORK
Operating System
Computer Memory
Bus
Input Devices
Central Processing Unit
Output Devices

HOW A CPU WORKS
INTRODUCTION
Function
Branching Instructions
Clock Pulses
Fixed-Point and Floating-Point Numbers

HISTORY
Early Computers
Transistor
The Integrated Circuit

HOW THE WEB WORKS


To access the Web, a user must have a computer connected to the Internet and appropriate software. The connection between the user's computer and the Internet can consist of a permanent, dedicated connection or a temporary, dial-up connection. A dial-up connection uses a modem to send data over the telephone system to another modem. It offers the lowest cost but requires the user to wait for the connection to be established each time the modem is used. A permanent connection uses a technology such as Asymmetric Digital Subscriber Line (ADSL, also known as DSL), a cable modem, or a dedicated leased circuit. It remains in place and is ready to use at all times. Permanent Internet connections cost more but offer higher capacity—that is, they can send more data at a faster speed.

Two pieces of software are needed to access the Web: (1) basic communication software that a computer uses to transfer data across the Internet and (2) a Web application program known as a browser that can contact a Web site to obtain and display information. Basic communication software, which is usually built into the computer's operating system, allows the computer to interact with the Internet. The software follows a set of protocol standards that are collectively known as TCP/IP (Transmission Control Protocol/Internet Protocol). Because it is built into the computer's operating system, TCP/IP software remains hidden from users. The software is invoked automatically by application programs that use the Internet.

The second piece of software needed for Web access consists of an application program known as a Web browser. Unlike basic communication software, a browser is directly visible to the user. To access the Web, the user must invoke the browser and enter a request. The browser then acts as a client. The browser contacts a Web server, obtains the requested information, and displays the information for the user.

Information on the Web is divided into pages, each of which is assigned a short identification string that is known as a Uniform Resource Locator (URL). A URL encodes three pieces of information: the protocol a browser should use to obtain the item, the name of a computer on which the item is located, including its domain name, and the name of the item. The domain name indicates whether the site is operated by a commercial or nonprofit business. For example, .com is a commercial site whereas .org is a nonprofit site. Many other domain names exist, including .edu for Web sites established by educational institutions.

In 2001 many other unique domain names were created. They comprised .info for informational sites, .biz for businesses, .name for individuals to register their name for a Web site or for an e-mail address, .museum for museums, .aero for the aviation industry, .coop for business cooperatives such as credit unions and electric coops, and .pro for professionals such as accountants, lawyers, and physicians. As of March 2002, all of these domain name suffixes were operational, with the exception of .pro.

Only the computer name is required in a URL. If the protocol is omitted, a browser assumes “http://,” and if the name of an item is omitted, the server chooses a page to send. Thus, the URL encarta.msn.com, which consists only of a computer name, is also valid.

Before it can obtain information, a browser must be given a URL. A user can enter the URL manually or click on a selectable link. In each case, once it has been given a URL, the browser uses the URL to obtain a new page, which it then displays for the user. The URL associated with a selectable link is not usually visible because the browser does not display the URL for the user. Instead, to indicate that an item is selectable, the browser changes the color of the item on the screen and keeps the URL associated with the link hidden. When a user clicks on an item that corresponds to a selectable link, the browser consults the hidden information to find the appropriate URL, which the browser then follows to the selected page. Because a link can point to any page in the Web, the links are known as hyperlinks. See also Hypermedia.

When a browser uses a URL to obtain a page, the information may be in one of many forms, including text, a graphical image, video, or audio. Some Web pages are known as active pages because the page contains a miniature computer program called a script or applet (a small application program). When a script or applet arrives, the browser runs the program. For example, a script can make images appear to move on the user's screen or can allow a user to interact with a mouse, keyboard, or microphone. Active pages allow users to play games on the Web, search databases, or perform virtual scientific experiments. Active pages are also used to generate moving advertisements, such as a banner that keeps changing or a logo that appears to rotate.

The codes that tell the browser on the client computer how to display a Web document correspond to a set of rules called Hypertext Markup Language (HTML). An HTML document consists of text with special instructions called tags, which are inserted to tell the browser how to display the text. The HTML language specifies the exact rules for a document, including the meaning of each tag. Thus, a person who creates an HTML page is responsible for inserting tags that cause the browser to display the page in the desired form. Not all Web pages use HTML. Graphics images are usually encoded using the Graphics Interchange Format (GIF) or Joint Photographic Experts Group (JPEG) standards. Active pages are written in a computer programming language such as ECMA Script or Java.

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