The Internet is a global dis'”tributed network of computers that use a common protocol to communicate-TCP/lP (Transmission Control Protocol/lnternet Protocol). TCP lIP is a simple protocol because it had to be implemented consistently on all computers and operating systems. Indeed TCP lIP is a truly universal protocol but you needn’t know much about it. It’s there when you need it and allows your computer to connect to any other computer on the Internet.
Each computer on the Internet has a unique address, for example 18.104.22.168. Every number is a value in the range 0 through 255 which means the Internet can’t have more than.256*256*256*256 or approximately 4 000 000 000 computers. This number isn’t as large as it may seem because many of the possible values are reserved. Others are assigned to organizations that mayor may not be using all of them. To accommodate a large number of users Internet service providers use a pool of addresses (since not all users connect at once, 256 addresses may accommodate 1000 users or more). It would be nice if we all had a unique IP address like an e-mail address, but this is not possible. If we did we-could build wide area networks that span the globe easily. However every time you connect to your Internet service provider you get a different IP address. If TCP lIP enables any two computers on the Internet to talk to each other why do we need another protocol? HTfP is the protocol of the Web. Whereas TCP lIP allows two computers to connect on the hardware level HITP is the language servers and clients use to exchange information. HTfP is optimized for requesting and supplying HfML documents. The Internet is more than the Web (although it’ seems the Web is taking over). To exchange files through the Internet for example computers use the File Transfer Protocol (FI’P). The protocol that is used depends on the type of information to be exchanged. All other protocols however run on top of the TCP/IP protocol.
In the following section I’m going to discuss briefly the components of the Web and the evolution of Web documents.
The simplest component of the Web is HTML. (HyperText Markup Language) . HIML is a simple language for formatting documents that are displayed in a Web browser. The primary task of the browser is to render documents according to the HTML tags they contain and display them on the monitor. Using HTML editors and WYSISYG (What You See Is What You Get) tools you can publish information in a form suitable for viewing with a browser on or off the Internet. There’s no reason that a company couldn’t publish its internal documents in HTML format and make them available on the local area network. The benefits a this approach are many and Office 97 supports HTML publishing. Any document you can create with an Office 97 application can be converted to HTML format and published on a Web server either on the Internet or ana corporate intranet. People can locate the information they seek by following hyperlinks a process they a already accustomed to. Information is up-to-date at least as much so as the person( in charge can make it.
HTML is made up of text-formatting tags that are placed in a pair of angle brackets they usually appear in pairs. The first tag turns on a formatting feature and matching tag turns it off. To format a few words in bold for example enclose the with the <B> and </B> tags as shown here:
Some <B>Words</B> in the sentence are formatted in <B>bold</B>
Of course not all tags are as simple. The <TABLE> tag for example which’is used to format tables requires additional tags like the <TR> tag, which delimits a new row in the table, and the <TO> tag, which delimits a new cell in a row. Tags are also assisted by attributes which are keywords with special meanings within a specific tag. The <A> tag which is used to insert a hyperlink in the document recognizes the HREP attribute. The syntax of a hyperlink to Microsoft’s home page on the Web would be something like
The text between the <A> and <./A> tags is marked as a hyperlink (displayed in a different color and underlined). The HREF attribute-in the <A> tag tells the browser which URL or address to jop to when a user clicks this hyperlink As a VB programmer you’ll have no problem picking up the syntax of HTML. If you are not faini liar with HTML you’ll want to peruse the quick introduction in the section II An HTML Primer” later in this chapter. It covers the elements of the language we are going to use in the examples in the following chapters and is addressed to VB programmers–just the essentials. For a complete HTML reference including all the tags recognized by Internet Explorer and their attributes
Server Client Interaction
A Web site consisting of HTML pages is interactive only in the sense that it allows the user to jump from page to page through hyperlinks. The client requests documents from the server and the server supplies them. In this simple interaction model which dominates the Web today, Web pages reside on the disks of the servers waiting to be requested by a client. Obviously, updating the information entails editing the HTML documents; no wonder most sites can’t provide up-to date information
The disadvantage of this model is that the client can’t engage in a conversation with the server so that information can flow in both directions. The development < . of gateway interfaces such as the Common Gateway Interface (CGl) has enabled Web authors to add dynamic content to the Web. The client can send specific requests to the server (e.g. “show me the invoices issued last month” or “show me the customers in North America”). The server doesn’t return a static page (a page that exists on the disk-and can be called by its name). Instead it executes a script or application that extracts “live” data from a database, formats the data as an HTML document and sends the document to the client. The client sees up-to-date accurate information.
The disadvantage of gateway programs is that they are difficult to implement and maintain. To simplify CGI programming Microsoft introduced several technologies the most recent and popular being Active Server Pages (a topic we’ll discuss in the last chapter of this book). Developing Active Server Pages is as simple as developing Visual Basic applications