The Internet

One way to think of the Internet is to picture it as a wide-area network that spans multiple geographic locations. Each location in this enormous network is comprised of a group of computers that are relatively close in proximity to each other and are connected by hardware and cabling.

Users communicate from one location to another using a communication protocol known as IP (Internet Protocol). This protocol, running on each computer connected to the Internet, ensures that communication breakdowns do not occur and that the networked computers can communicate and exchange data properly with each other.

Every computer connected to the Internet has a unique IP address. If duplicate IP addresses existed, information using a given address could end up in the wrong place. It would be like using the same street address for two residences. As a sender of information, you would never know if the intended receiver did receive your information.

Internet applications

Software applications that run on the Internet are known as Internet applications. The following table lists some of the most popular Internet applications:

Internet applications communicate across the Internet by using IP. IP transmits application data in small packets to a destination IP address. The receiving host processes the information that it receives.

Intranet applications

An intranet is a private LAN (Local Area Network) or WAN (Wide Area Network) that lets you use and interact with your Internet-based applications in a secure environment. These private networks exist in large corporations, small companies, and even home offices. Private networks let companies and organizations determine who can share their information and who can access it.

An intranet application is an application that works on a private intranet (network). It differs from an Internet application only in who can access it and the location of the client computer accessing it. An intranet application can also operate over a public Internet. When an intranet application runs on the Internet, the application is called an Internet application. These terms, Internet applications and intranet applications, are used interchangeably throughout this book.

Web servers

A web server is a software program that serves web pages to requesting clients. The web server software runs on any computer. Often people refer to the host running the web-server software as the web server, and think of it as the hardware. However, technically, the web server is just the software program and not the hardware.

How a web server and connecting hosts communicate

When a user at a specific IP address requests a file, the web server retrieves that file and returns it to the requesting IP address. The contents of a file are not important to the web server. It is the web browser that makes the request and interprets and displays the data in the file that was returned from the web server.

When you make a request from a web server, an IP connection is made across the Internet between the client making the request and the host running the web-server software. As soon as a request is satisfied by the web server, the Internet connection between the client and the host breaks. A page containing images or links to other pages all require separate connections. Often, it takes many requests to retrieve all the information on one web page.

Web pages

The information on the World Wide Web is presented in web pages. You can create web pages using a series of client-side technologies. A web page can include a variety of information: text, lists, forms for capturing data, tables for presenting data, scripts that perform a function, multimedia content that animate pages, and so on.

No matter the content of the web page, the web browser must process and display the page.

Web browsers

A web browser is a software program residing on a computer that you use to view pages on and navigate the World Wide Web. When you use a browser to request a page on a website, that browser is making a web connection to a web server.

As mentioned previously, the web browser processes the web pages that it receives from a web server and displays the pages to the user. Depending on the browser that you use and the features it includes, you might be able to play multimedia files, view and interact with Java applets, read your e-mail, or use other advanced features.

Some of the most popular web browsers today are Microsoft Internet Explorer, Netscape Navigator, and Mozilla. Unfortunately, most browsers today parse web pages differently. Web designers must pay special attention to the way a browser behaves, or users might not see the pages as the designers intended. Therefore, web designers test their pages on multiple browsers before publishing them on their website.

HTML

Web page authors create plain text files using the Hypertext Markup Language. This language, known as HTML, consists of a series of simple-to-learn tags. You can use these tags to mark up a page of text. Tags can indicate page elements, structure, formatting, hypertext links to other locations, and so on. Web browsers read the HTML tags and format the text and styles that appear on the computer screen.

HTML tags usually have a starting and ending tag, surrounding the text that they affect. The starting tag turns on a feature (heading, bold, and so on), and the ending tag turns it off. All ending tags have a forward slash (/) preceding the tag name.

Most HTML tags look like this:

<TheTagName>text</TheTagName>

The tag name is always enclosed in angle brackets (<>) and is case-insensitive, which means that you can specify the tag name in uppercase, lowercase, or mixed case.

Most web browsers let you view the source of an HTML page. This option is usually located in a menu or a button.

JavaScript

Web developers write JavaScript to create small programs that run in the browser. JavaScript is one of the most popular client-side scripting languages today. It is supported by almost all browsers on the market. Web developers use JavaScript to do these actions:

• Validate user actions.
• Create scrolling messages in a browser's status bar.
• Animate text or images.

JavaScript can be inserted in the HTML file. HTML uses tags to mark the start and end of the code. The <script> tag tells the browser that the following chunk of text, bounded by the closing </script> tag, is not HTML, but rather script code to be processed.

Although using JavaScript seems much like inserting HTML content, JavaScript is more difficult to learn than HTML. For more information about JavaScript, see any JavaScript primer.

URLs

Every piece of information on the World Wide Web has a unique address. This address is called a Uniform Resource Locator, or URL. A URL is a pointer to some bit of data on the web. This information might include a web document, a file on a FTP site, a posting on Usenet, or even an e-mail address. URLs contain information about the following:

• How to get the information (what protocol to use: FTP, HTTP, and so on)
• The Internet host name to contact (for example, www.macromedia.com; http://localhost/mysite; or ftp.mysite.com)
• The directory or other location to locate the requested information

In addition, you use special URLs to send e-mail and for using the Telnet program.

Understanding web application servers

As explained previously, web browsers make requests, and web servers fulfill those requests by returning the requested information to the browser. This information is usually HTML files, as well as other types.

When you think about it, web servers’ capabilities are limited because all they do is wait for requests to arrive and attempt to fulfill those requests as soon as possible. Web servers do not let you do the following tasks:

• Interact with a database.
• Serve up customized information based on user preferences or requests.
• Validate user actions.

Web servers, basically, locate information and return that information to a web browser.

To extend the capabilities of a web server, you need a web application server. A web application server is a software program that lets the web server do more tasks, like those listed in the previous paragraph.

How a web server and web application server work together

The following steps explain how a web server processes a page that also needs processing by a web application server:

1. The user requests a page by typing a URL in a browser, and the web server receives the request.
2. The web server looks at the MIME type (or file extension) to determine whether a web application server must process the page. Then one of the following actions occur:

• If the MIME type indicates that the file is a simple web page (typically an HTM extension), then the web server fulfills the request and sends the file to the browser.
   

• If the MIME type indicates that the requested file is a page that a web application server must process (CFM or CFC extension for ColdFusion requests), then the web server passes it to the web application server. The web application server processes the page and sends the results to the web server, which returns those results to the browser. The following figure shows this process:

Click here for diagram

Web application servers process code in a page that a browser and web server cannot interpret. The web server recognizes these requested pages by the file extension and forwards it to the web application server for action. The web application server interprets the programming instructions in the page and generates output that a web browser can interpret. Then the web server returns the output to the browser.

By using a web application server, web developers can build highly interactive and data-rich sites, such as the following:

• Create shopping carts and e-commerce sites.
• Query other database applications for data.
• Dynamically populate form elements.
• Respond with e-mail immediately after a user submits a form.

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