 | HTML & XHTML: The Definitive Guide, 4th Edition |  |
Every computer connected to the Internet (even a beat-up old Apple II) has a unique address: a number whose format is defined by the Internet Protocol (IP), the standard that defines how messages are passed from one machine to another on the Net. An IP address is made up of four numbers, each less than 256, joined together by periods, such as 192.12.248.73 or 131.58.97.254.
While computers deal only with numbers, people prefer names. For this reason, each computer on the Internet also has a name bestowed upon it by its owner. There are several million machines on the Net, so it would be very difficult to come up with that many unique names, let alone keep track of them all. Recall, though, that the Internet is a network of networks. It is divided into groups known as domains, which are further divided into one or more subdomains. So, while you might choose a very common name for your computer, it becomes unique when you append, like surnames, all of the machine's domain names as a period-separated suffix, creating a fully qualified domain name.
This naming stuff is easier than it sounds. For example, the fully qualified domain name www.oreilly.com translates to a machine named "www" that's part of the domain known as "oreilly," which, in turn, is part of the commercial (com) branch of the Internet. Other branches of the Internet include educational institutions (edu), nonprofit organizations (org), U.S. government (gov), and Internet service providers (net). Computers and networks outside the United States may have a two-letter abbreviation at the end of their names: for example, "ca" for Canada, "jp" for Japan, and "uk" for the United Kingdom.
Special computers, known as name servers, keep tables of machine names and their associated unique IP numerical addresses and translate one into the other for us and for our machines. Domain names must be registered and sometimes paid for through the nonprofit organization InterNIC. Once registered, the owner of the domain name broadcasts it and its address to other domain name servers around the world. Each domain and subdomain has an associated name server, so ultimately every machine is known uniquely by both a name and an IP address.
The Internet connects two kinds of computers: servers, which serve up documents, and clients, which retrieve and display documents for us humans. Things that happen on the server machine are said to be on the server side, while activities on the client machine occur on the client side.
To access and display HTML documents, we run programs called browsers on our client computers. These browser clients talk to special web servers over the Internet to access and retrieve electronic documents.
Several web browsers are available -- most are free -- each offering a different set of features. For example, browsers like Lynx run on character-based clients and display documents only as text. Then there are others that run on clients with graphical displays and render documents using proportional fonts and color graphics on a 1024 x 768, 24-bit-per-pixel display. Others still -- Netscape Navigator, Microsoft's Internet Explorer, Opera, and Mozilla, to name a few -- have special features that allow you to retrieve and display a variety of electronic documents over the Internet, including audio and video multimedia.
All web activity begins on the client side, when a user starts his or her browser. The browser begins by loading a home page document from either local storage or from a server over some network, such as the Internet, a corporate intranet, or a town extranet. In these latter cases, the client browser first consults a domain name system (DNS) server to translate the home page document server's name, such as www.oreilly.com, into an IP address, before sending a request to that server over the Internet. This request (and the server's reply) is formatted according to the dictates of the Hypertext Transfer Protocol (HTTP) standard.
A server spends most of its time listening to the network, waiting for document requests with the server's unique address stamped on it. Upon receipt, the server verifies that the requesting browser is allowed to retrieve documents from the server, and, if so, checks for the requested document. If found, the server sends (downloads) the document to the browser. The server usually logs the request, the client computer's name, document requested, and the time.
Back on the browser, the document arrives. If it's a plain-vanilla ASCII text file, most browsers display it in a common, plain-vanilla way. Document directories, too, are treated like plain documents, although most graphical browsers will display folder icons, which the user can select with the mouse to download the contents of subdirectories.
Browsers also retrieve binary files from a server. Unless assisted by a helper program or specially enabled by plug-in software or applets, which display an image or video file or play an audio file, the browser usually stores downloaded binary files directly on a local disk for later use.
For the most part, however, the browser retrieves a special document that appears to be a plain text file, but contains both text and special markup codes called tags. The browser processes these HTML or XHTML documents, formatting the text based upon the tags and downloading special accessory files, such as images.
The user reads the document, selects a hyperlink to another document, and the entire process starts over.
We should point out again that browsers and HTTP servers need not be part of the Internet's World Wide Web to function. In fact, you never need to be connected to the Internet, an intranet or extranet, or to any network, for that matter, to write documents and operate a browser. You can load up and display on your client browser locally stored documents and accessory files directly. This isolation is good: it gives you the opportunity to finish, in the editorial sense of the word, a document collection for later distribution. Diligent authors work locally to write and proof their documents before releasing them for general distribution, thereby sparing readers the agonies of broken image files and bogus hyperlinks.[3]
[3]Vigorous testing of the HTML documents once they are made available on the Web is, of course, also highly recommended and necessary to rid them of various linking bugs.
Organizations, too, can be connected to the Internet and the World Wide Web, but also maintain private webs and document collections for distribution to clients on their local network, or intranet. In fact, private webs are fast becoming the technology of choice for the paperless offices we've heard so much about these last few years. With HTML, and especially with next-generation XHTML document collections, businesses and other enterprises can maintain personnel databases, complete with employee photographs and online handbooks, collections of blueprints, parts, and assembly manuals, and so on -- all readily and easily accessed electronically by authorized users and displayed on a local computer.
Like many popular technologies, HTML started out as an informal specification used by only a few people. As more and more authors began to use the language, it became obvious that more formal means were needed to define and manage -- to standardize -- the language's features, making it easier for everyone to create and share documents.
The World Wide Web Consortium (W3C) was formed with the charter to define the standards for HTML. Members are responsible for drafting, circulating for review, and modifying the standard based on cross-Internet feedback to best meet the needs of the many.
Beyond HTML, the W3C has the broader responsibility of standardizing any technology related to the World Wide Web; they manage the HTTP, Cascading Style Sheet, and Extensible Markup Language (XML) standards, as well as related standards for document addressing on the Web. And they solicit draft standards for extensions to existing web technologies.
If you want to track HTML, XML, XHTML, CSS, and other exciting web development and related technologies, contact the W3C at http://www.w3.org .
Also, several Internet newsgroups are devoted to the Web, each a part of the comp.infosystems.www hierarchy. These include comp.infosystems.www.authoring.html and comp.infosystems.www.authoring.images.
Even broader in reach than W3C, the Internet Engineering Task Force (IETF) is responsible for defining and managing every aspect of Internet technology. The World Wide Web is just one small part under the purview of the IETF.
The IETF defines all of the technology of the Internet via official documents known as Requests For Comment, or RFCs. Individually numbered for easy reference, each RFC addresses a specific Internet technology -- everything from the syntax of domain names and the allocation of IP addresses to the format of electronic mail messages.
To learn more about the IETF and follow the progress of various RFCs as they are circulated for review and revision, visit the IETF home page, http://www.ietf.org.
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| 1. HTML, XHTML, and the World Wide Web |  | 1.3. HTML: What It Is |
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