Internet is a worldwide and opening interconnected computer network. It is a special network, on which users can administer themselves. Internet is based on TCP/IP protocol. It can provide various information resources and many network services for network users. Basic functions of Internet include: sending and receiving e-mail, issuing news, Telnet, file transfer, information access, etc. Internet has already become the widest influential international linking network with the largest scale and the most users.
It's really a lot simpler than you guess. A network is a collection of computers that are connected together so they can share information, and the Internet is a network of networks. It lets individuals on one network share information with users on another network that may be thousands of miles away. The shared information can take many forms. For instance, you can use the Internet to send e-mail message, or to download files, or to view video clips, or to listen to music. You can even use the Internet for banking and shopping. It's a lot like the phone system, except that instead of just talking you can exchange all different kinds of information.
The physical Internet looks like a vast net of wires. A few high-speed “backbone” cables branch out into other cables, which in turn radiate outward into finer strands. Most of the developed areas of the world have already been wired, so by routing your information or requests through this vast system you can reach other Internet users all over the world. Fortunately the routing happens automatically, just as it does with telephone calls, so you don't need to know how data gets from your computer to a computer in Timbuktu.
And the system is democratic. Anyone can use it. You don't even need to have your own network in order to connect you can communicate using the current infrastructure by plugging into somebody else's network. Maybe you're plugging into somebody else's network. Maybe you are already doing this. If you have a Serial Line Internet Protocol (SLIP) or Point-to-Point Protocol (PPP) account, you simply connect via modem and phone line to your access provider network site (known as a “point of presence”). The actual transmission of data is free, but you'll probably need to pay the access provider for the time you spend accessing its equipment.
There are hundreds of thousands of users sitting at their PCs right now, gathering information from far-flung reaches of the globe. Pretty soon the Internet will be accepted as a simple fact of modern life, and people will forget how it originated.
And the following will tell you in brief, how it originated.
As with many technological advances, the Internet began as a military research project. It was the early 1970s, and the U.S. Defense Department implemented a network, called ARPAnet, which was designed so that even if part of its physical structure were destroyed, information could still be sent to any remaining destination.
In the early 1980s, local area networks at a variety of research institutions started hooking into ARPAnet. They were able to do this by using the same underlying technology that ARPAnet used to transfer data and make sure it got to the right place.
Then in the late 1980s, the National Science Foundation (NSF) established five supercomputer centers and connected them via their own network, NSFnet. Based on the same technology as ARPAnet, the site were physically linked by special phone lines. Since the phone companies charged by the mile for these lines, it was very expensive to gather information from one of these centers. The NSF decided to create several regional networks with dozens of connected sites; any data could be passed along the line from one node to another, saving on the costly line charges.
The rest of the Internet's history is one of expansion. As new educational institutions and government agencies developed their own networks and joined NSFnet, the original wiring quickly became overloaded. Much faster line were put in, faster computers were installed, and what is now known as the Internet opened its virtual doors to most of the academic and government community. Universities from other nations, not to mention foreign governments, started joining the Internet. In the early 1990s, commercial organizations began to jump on board, and it is now common for companies to promote, and even sell, all sort of products and services over the Internet.
（1）Internet Address.
On the Internet, the word address always refers to an electronic address, not a postal address. If a computer user asks for your “address”, he or she wants your Internet address.
Internet addresses all follow the same form: the person's user-id, followed by an @ character (the “at” sign) followed by the name of a computer. (Every computer on the Internet has a unique name). Here is a typical example:
In this case, the user-id (pronounced “user-eye-dee”) is frest, and the name of the computer is ibm.com.cn. As this example shows, there are never any spaces within an address .
You know that each person has a user name called a user-id. It is this user-id that we use as the first part of someone's address. If you have a UNIX system, your user-id will be the name that you use to log in.
The part of the address after the @ character is called the domain. In this case, the domain is ibm.com.cn.Thus, the general form of all Internet addresses is:
As you might imagine, a user-id by itself is not necessarily unique. For instance, within the entire Internet, there are probably a number of people lucky enough to have a spiffy user-id like frest.
What must be unique is the combination of user-id and domain. So, although there may be more than one frest on the Internet, there can be only one such user-id on the computer named ibm.com.cn.
If you read an Internet address out loud, you will see that using the @ character is appropriate. For example, let's say that you want to send mail to the person at the address we just mentioned. The command to do so is:
As you enter this command, you can say to yourself, I am sending mail to frest, who is at the computer named “ibm.com.cn”.
Sometimes an address in this form is called a fully qualified domain name or FQDN.
We call each part of a domain a sub-domain. Sub-domains are separated by periods. In our example, there are three sub-domains: ibm, com, and cn.
The way to understand a domain name is to take at the sub-domains from right to left. The name is constructed so that each sub-domain tells you something about the computer. The rightmost sub-domain, called the top-level domain, is the most general. As you read to the left, the sub-domains become more specific.
In our example, the top-level domain “cn” tells us that the computer is located in China. The next sub-domain “com” tells us it is a company. And the next sub-domain “ibm” tells us the name of the company (IBM, a large-scale company). Thus, as you enter the command:
You can say more clearly to yourself, “ I'm sending to frest, at a computer named ibm, which is a company located in China.”
（2）DNS—Domain Name System.
Programs rarely refer to hosts, mailboxes, and other resources by their binary network addresses. Instead of binary numbers, they use ASCII strings, such as frest@ibm.com.cn. Nevertheless, the network itself only understands binary addresses, so some mechanism is a required to convert the ASCII strings to network addresses. In the following sections we will study how this mapping is accomplished in the Internet.
When thousands of workstations were connected to the Internet, everyone realized that this approach could not continue to work forever. For one thing, the size of the file would become too large. However, even more importantly, host name conflicts would occur constantly unless names were centrally managed, something unthinkable in a huge international network. To solve these problems, DNS (the Domain Name System) was invented.
The essence of DNS is the invention of a hierarchical, domain-based naming scheme and a distributed database system for implementing this naming scheme. It is primarily used for mapping host names and e-mail destinations to IP addresses but can also be used for other purposes. DNS is denned in RFCs 1034 and 1035.
Very briefly, the way DNS is used as follows. To map a name onto m IP address, an application program calls a library procedure called the resolver, passing it the name as a parameter. The resolver sends a UDP packet to a local DNS server, which then looks up the name and returns the IP address to the resolve, which then returns it to the caller. Armed with the IP address, the program can then establish a TCP connection with the destination, or send it UDP packets.
The top-level domains come in two flavors: generic and countries. The generic domains are com (commercial), edu (educational institutions), gov (the government), int (certain international organizations), mil (the U.S. armed forces), net (network providers), and org (nonprofit organizations). The country domains include one entry for every country, as denned in ISO 3166.
Each domain is named by the way upward from it to the (unnamed) root. The components are separated by periods (pronounced “dot”). Thus Sun Microsystems engineering department might be eng.sun.com, rather than a UNIX-style name such as /coom/sun/eng.
Domain names are case insensitive, so edu and EDU mean the same thing. Component names can be up to 63 characters long, and all path names must not exceed 255 characters.
（3）HTTP and SMTP.
①HTTP—Hyper Text Transfer Protocol.
The standard Web transfer protocol is HTTP (Hyper Text Transfer Protocol). Each interaction consists of one ASCII request, followed by one RFC 822 MIME-like response. Although the use of TCP for the transport connection is very common, it is not formally required by the standard. HTTP is constantly evolving. Several versions are in use and others are under development.
The HTTP protocol consists of two fairly distinct items: the set of requests from browsers to servers and the set of responses going back the other way. And all the newer versions of HTTP support two kinds of requests: simple requests and full requests.
②SMTP—Simple Mail Transfer Protocol.
Within the Internet, e-mail is delivered by having the source machine establish a TCP connection to port 25 of the destination machine. Listening to this port is an e-mail daemon that speaks SMTP (Simple Mail Transfer Protocol). This daemon accepts incoming connections and copies messages from them into the appropriate mailboxes. If a message cannot be delivered, an error report containing the first part of the undeliverable message is returned to the sender.
SMTP is a simple ASCII protocol. After establishing the TCP connection to port 25, the sending machine, operating as the client, waits for the receiving machine, operating as the server, to talk first. The server starts by sending a line of text giving its identity and telling whether or not it is prepared to receive mail. If it is not, the client releases the connection and tries again later.
Even though the SMTP protocol is well denned (by RFC 821), a few problems can still arise. One problem relates to message length. Some older implementations cannot handle messages exceeding 64 KB. Another problem relates to timeouts. If the client and server have different timeouts, one of them may give up while the other is still busy, unexpectedly terminating the connection. Finally, in rare situations, infinite mail-storms can be triggered. For example, if host 1 holds mailing list A and host 2 holds mailing list B and each list contains an entry for the other one, then my message sent to either list will generate a never-ending amount of e-mail traffic.
The software that supports the Internet provides a large number of technical services up on which everything else is built. Most of these services operate behind the scenes, and you do not need to understand them.
Nevertheless, there are four important Internet services we do need to talk about. You don't need to know the details, but you need to know they exist.
First, the mail service reliably transmits and receives messages. Each message is sent from one computer to another on its way to a final destination. Behind the scenes, the mail service ensures that message arrives intact at the correct address.
The next service, called Telnet, allows you to establish a terminal session with a remote computer. For example, you can use Telnet to connect to a host on the side of the world. Once the connection is made, you can log in to that computer in the regular manner. (Of course, you will need a valid user account and password.) Telnet also allows two programs to work cooperatively by exchanging data over the Internet.
The word “telnet” often used as a verb. For example, you might tell someone, “If you telnet to this computer, you will be able to use a computerized reference book”.
The third service is called FTP (File Transfer Protocol). FTP allows you to transfer files from one computer to another. Most of the time, you will use FTP to copy a file from a remote host to your computer. This process is called downloading. However, you can also transfer files from your computer to a remote host. This is called uploading. In addition, you should find it necessary, FTP will allow you to copy files from the remote host to another.
The last Internet service you should understand is the general client/server facility.
Now let's say a few words about the client/server. We know that one of the principal uses of a network is to allow the sharing of resources. Much of the time, this sharing is implemented by two separate programs, each running on different compute. One program, called the server, the other program, called client, makes use of the resource.
For instance, say that you are working with a word processing program that is running on your own PC. You tell the program that you want to edit a particular file that is stored on another computer on your network. Your program will pass a message to that computer asking it to send the file. In this case, your word processing program is the client while the program that accepts the request and sends the file is the server. More precisely, it is a file server.
We expect other drivers to observe the rules of the road and the same is true as we travel through cyberspace.
To personalize your messages, you can use smiles, also known as emotions expressions you create from the characters on your keyboard. A few popular ones are shown in Table 3-3.
Table 3-3 A few popular emotions
|
Emotion |
Meaning |
Emotion |
Meaning |
|
:-) |
Happy |
:-e |
Disappointed |
|
:-( |
Sad |
:- |
Mad |
|
:-@ |
Screaming |
:-D |
Laughing |
|
:-o |
Surprised |
;-) |
Winking |
|
:-I |
Indifferent |
|
|
Keep your communications to the point. Some people pay for Internet access by the hour. The longer it takes to read your messages, the more it may cost them. This is true whether you post messages to a newsgroup or a mailing list.
Keep in mind that anything you post to a newsgroup or type into a chat session is a public comment. You never know who's reading it, or who may copy it and spread it around.
If you're posting a message to a public bulletin board, forum, or newsgroup, stick to the topic. Don't make the mistake of indiscriminately posting advertisements to every newsgroup you can think of. This practice, referred to as spamming, will quickly lead to another unpleasant Internet practice known as naming. What is naming? Sometimes you might offend someone unintentionally. Be prepared to receive some angry e-mail or be treated rudely in a public discussion. This is called being named. If you attack back, you will spark what is known as a name war. To contain the heat, the best response usually is no response at all.
If you post an ad to a newsgroup, or send it in an e-mail, clearly identify it in the subject line. That way people who aren't interested can delete it.
To keep messages short, there are some abbreviations you can:
· <BTW> means “ by the way ”.
· A <G> enclosed in brackets indicates grinning.
· A good one to keep handy in case you're worried about offending someone is <IMHO> —In My Humble opinion.
· One of our favorites is <ROTFL>, which stands for Rolling on the Floor Laughing.
Netiquette isn't something you learn overnight, so don't let your fear of not knowing cyber-protocol hold you back.
Keep in mind that FAQs (Frequently Asked Questions) are very handy documents to read before asking questions. You should always consult them whenever they are available.
The Internet is a strange and wonder all network that has made it possible for people all around the world to connect with each other in meaningful ways. Whether for research, education, business, or fun, the Internet has changed how many of us live, work, and play, in ways we may not even be fully aware of.
As the Internet continues to evolve, so do the issues that impact the way we use it. Whether you are the consummate hacker or just an occasional driver on the information highway, you play a role in determining the future direction of this road. From privacy, security, and freedom of speech to honesty and consideration in the way we interact with others, we all have a respon-
sibility to preserve and protect its unique character.
|
Internet |
互联网 |
|
TCP/IP |
传输控制协议/网际协议 |
|
Telnet |
远程登录 |
|
Serial Line Interface Protocol (SLIP) |
串行线路接口协议 |
|
|
电子邮件 |
|
NSFnet |
国家科学基金网 |
|
domain |
域 |
|
User-id |
用户标识符 |
|
DNS |
域名系统 |
|
UDP |
用户数据报协议 |
|
FTP |
文件传输协议 |
|
terminal session |
终端会话 |
|
HTTP |
超文本传输协议 |
|
SMTP |
简单邮件传输协议 |
|
port |
端口 |
|
e-mail daemon |
电子邮件后台程序 |
（1）Telnet（远程登录）。在网络通信协议telnet的支持下，使用户的计算机为某远程计算机的仿真终端，共享远程计算机的软硬件资源、数据库和Internet的其他信息服务。
（2）e-mail（电子邮件）。电子邮件是Internet上使用最广泛和最受欢迎的服务，它是用户之间进行快速、简便、可靠且低成本联络的现代通信手段。
（3）modem（调制解调器）。如果通信信道不允许直接传输计算机所产生的数字信号, 就需要在发送端将数字信号变换成模拟信号（称为调制），再在接收端将模拟信号还原成数字信号（称为解调）。
（4）DNS（Domain Name Systems，域名系统）。IP地址和域名都能表示一台连到Internet上的计算机，而把域名翻译成IP地址的工作由域名服务器（DNS）完成，在Internet上有许多域名服务器负责翻译地址。
（5）FTP（File Transfer Protocol，文件传输协议）。在网络上的计算机之间传输文件的协议。用户可以将本地计算机上的文件传送到远程主机上（upload称为上传），也可从远程主机上获取所需要的文件（download称为下载）。
（6）HTTP（Hyper Text Transfer Protocol，超文本传输协议）。是为分布式超媒体信息系统设计的面向对象的传输协议。
（7）SMTP（Simple Mail Transfer Protocol，简单邮件传输协议）。它是描述客户机与远程主机之间传送电子邮件的协议。
（8）port（端口）。在计算技术和通信技术中，网点上的一种功能部件，通过它数据可进入或离开一个数据网络或计算机。
（9）e-mail daemon（电子邮件后台程序）。daemon是指Internet中用于邮件收发的后台程序。
（10）newsgroup（新闻组）。新闻组为每个网络新闻组定义了一个独特的名字，用户想加入一个新闻组并参与其中的讨论时，只需输入新闻组的名字。
（11）bulletin board（公告板）。电子公告板是Internet上最知名的信息服务之一，它允许用户选择自己感兴趣的话题，阅读有关的消息，也可以“张贴”自己的信息供他人阅读，还可以发表对其他人观点的评论。
（12）由字符组成的图释，也称做Smiley，由emotion和icons合成。
1. Fill in the following blanks.
（1）Internet is a worldwide and interconnected computer network.
（2）DNS is primarily used for mapping host names and e-mail destinations to .
（3）On the Internet, the word address always refers to an address, not a postal address.
（4）SMTP is a simple protocol.
2. Write out the full text of the following abbreviations in English.
（1）SLIP
（2）PPP
（3）User-id
（4）DNS
（5）UDP
（6）FTP
（7）HTTP
（8）SMTP
3. Translate the following words into Chinese.
（1）binary
（2）request
（3）response
（4）port
（5）e-mail daemon
（6）server
（7）service
（8）terminal session
（9）Internet
（10）TCP/IP
（11）Telnet
（12）source machine
（13）destination machine
How TCP/IP Works
Over the past few years, one of the most written－about network topics has been IP. But even with all this attention, few, if any, stories have traced the protoco's basic workings, that is, how routers and Layer 3 switches act upon IP information to move Ethernet packets across the network.
As a point of referenced, bear in mind that IP is a member of the TCP/IP protocol suite.
TCP functions at the Open Systems Interconnection (OSI) transport layer, or Layer 4.Its chief responsibility is to ensure reliable end-to-end connectivity. IP, located one layer down, at the OSI network layer, or Layer 3, communicates the addresses of each packet's sender and receiver to the routers along the way. Routers and Layer 3 switches can read IP and other Layer 3 protocols. This information, combined with routing tables and other network intelligence, is all it takes to get across the room or around the world via TCP/IP.
The routing process begins with an IP address that is unique to the sending end station .End stations may be assigned permanent IP addresses or they may borrow them as needed from a Dynamic Host Configuration Protocol（DHCP）server or other service.
Each packet carries a source address, which under current（IPv4）specifications is 32-bit long In its header, each packet also carries the IP address of the final destination.
If the sending end station determines that the destination address is not local, the packet goes to a first-hop router, typically one that is close and has been reassigned to the sender.
The router inspects the packet's IP address and performs a route table lookup to see if the destination end station resides on the local（physically connected）network, typically called an IP subnet .An IP subnet usually is assigned to each of the router's network interfaces.
If the destination IP address is local, the router searches an internal store of IP addresses and local-device Media Access Control（MAC）addresses. This store is known as the Address Resolution Protocol（ARP）cache. ARP is the universal tool for matching IP addresses to MAC addresses. If the destination's MAC address appears, the router installs that MAC address in the packet header（removing its own MAC address because that's no longer needed）and sends the packet to the destination end station.
In the event that the destination MAC address does not appear in the ARP cache-it might have timed out, for instance-the router must broadcast an ARP request to the subnet referenced by the packet's destination IP address. The end station with that IP address responds, sending back its MAC address. The router updates its cache, installs the new MAC address into the packet header and launches the packet.
If the route table lookup shows that the packet is destined for a non-local subnet, the router forwards the packet to the next-hop router using the next-hop router's MAC address. Routing tables are continuously built and rebuilt by intelligent discovery protocols, such as Routing Information Protocol or Open Shortest Path First（OSPF）. Each router's routing table shows the best route to the destination address; for addresses that may be several hops away, it shows the best next-hop router.
Java Technology
“Write Once, Run Anywhere”. That's the trademark answer. But what does that answer really mean?
The Java technology is an object-oriented, platform-independent, multithreaded programming environment. It is the foundation for smart Web and networked services and allows you to securely extend your enterprise through platform independence. All kinds of systems can talk to each other, from smart cards to supercomputers, regardless of the underlying hardware or system software.
When software written in the Java programming language is compiled with Java technology, byte code results. The Java virtual machine can explain or interpret that byte code to any platform on which the Java virtual machine is installed. This means no more porting of programs to platforms.
Look at it this way. Suppose you speak only English. And suppose that you are speaking at an international conference attended by people like you—who speak only their native languages. Should you hire interpreters for each language represented? Should you supply English/mumble dictionaries to each attendee? Those solutions are costly, time-consuming, and error prone. But suppose you discover a language that can be understood by anyone wearing a special and inexpensive earphone that contains a universal interpreter that makes the new language instantly intelligible—no language barriers whatsoever. You'd learn that language and order up the earphones. This interpretable language is analogous to the Java technology, and the universal interpreter to the Java virtual machine, which converts the code to the correct semantics.
That’s how Java technology takes programming to a new meeting of the minds, where an interpretation process resolves issues that arise from different operating systems and platforms.
Internet bandwidth will continue to grow exponentially over the next 10 to 15 years. This phenomenon creates business opportunities, which will impact our lives in ways we hadn't previously imagined. With this increase come challenges. How does one expand with the associated complexities?
The “Write Once, Run Anywhere” capabilities of the Java platform position you to address this exponential growth. At Sun, we call this “taking it to the nth”, utilizing the openness and flexibility of Java technology as the foundation of the Sun Open Net Environment architecture our solution for creating, assembling, and deploying Web-based services and more.
The Java platform is a fundamentally new way of computing based on the power of net-
works and the idea that the same software should run on many different kinds of computers, consumer gadgets and other devices.
With Java technology, you can use the same application from any kind of machine—a PC, a Macintosh computer, a network computer, or even new technologies like Internet screen phones.
（1）It works everywhere.
The idea is simple. Java technology-based software works just about everywhere—from the smallest devices to supercomputers. Java technology components don't care what kind of computer, phone, TV or operating system they run on. They just work on any kind of compatible device that supports the Java platform.
Java technology is widely regarded as revolutionary, because it was designed to let computers and devices communicate with one another much more easily than ever before.
（2）Want to see some Java technology-based software.
Perhaps the most visible examples of Java technology-based software today are on the Internet and on enterprise networks. They're nimble, interactive programs called “applets”. Applets work inside Web browsers on computers and other devices.
And there are other kinds of Java technology-based software. Programs written in the Java programming language can run directly on your computer without requiring a browser, or on servers, on large mainframe computers, or other devices.
For example, Java technology-based software running on servers in large companies monitors transactions and ties together data from existing computer systems. Other companies are using Java technology-based software on their internal web sites to streamline communication and the flow of information between departments, suppliers and customers.
Why is Java technology so important? It’s the network!
With Java technology, the Internet and private networks become your computing environment. Coupled with the power of networking, the Java platform is helping computer users to do things that were previously unimaginable. For example, users can securely access their personal infor-
mation and applications when they're far away from the office by using any computer that's connected to the Internet；soon they'll be able to access tailored applications from a mobile phone based on the Java platform, or even use smart cards as a pass key to everything from the cash machine to ski lifts.
Why Java technology? Networks require software that is portable, modular, and secure—all areas where Java technology shines, because it was designed for use on networks from the beginning.
As you'll see on the next few pages, businesses are using Java technology because it connects easily to existing computing systems, lowers computing costs and speeds software development. It also lets businesses use the Internet to securely connect to their customers, suppliers and partners. And consumers benefit from Java technology because it brings personal, business and entertainment services to them—easily and securely—in many locations and on many different kinds of appliances and devices at home, at work and on the road.
So, what does the Java platform really do? It simplifies computing both for users and for the companies building and using computers and software.
Java technology addresses many of today's most pressing business computing problems—
complexity, incompatibility and security. It has proved invaluable in opening new business opportunities.
The Java platform's ease of development and widespread industry support mean lower development costs and quicker time to market. Built-in security protects company's information and assets. The lightweight distributed model eliminates software installation headaches and lowers administrative and maintenance costs of managing a computing network, that is, “total cost of ownership”, or TCO. Platform independence flees you to pick the hardware and operating system best suited to their needs.
And because Java technology programs can run on just about any type of computer and many devices, a user's applications and data are accessible from network computers on the factory floor, laptops or other networked devices on the road or in the field.
Enterprises like Home Depot, Xerox, CAX, NASA’s Jet Propulsion Laboratory, and Kaiser Permanente are discovering the Java software is not a replacement technology, but rather an extension of their current computing environments.
If you're a developer, the platform independence of the Java programming language means that you can write a program once, and have it run on dozens of different kinds of machines.
Java programming language software is scalable. For instance, the commerce software and services you create for a set-top box running the Personal Java platform can also be made available instantly on network computers or web browsers on desktop computers. The same Java technology program will work on all three devices without modification. And with proper design, it's possible to write an application that call run on an almost infinite array of devices.
The open, platform-independent and object-oriented nature of Java technology means developers can solve the problem of integrating with existing computers that previously would have seemed unthinkably complex. As a bonus, most Java programming language software developers report that Java programming language software is easier to create and maintain, when compared with traditional languages such as C and C++.
A detailed technical explanation of Java technology written for developers is contained in the Java Language Environment White Paper.
Java technology will soon be integrated into many aspects of your life. Interactive Internet services will be available not only on personal computers, but also on appliances throughout the household and on the road. Suppose home banking, Internet shopping, entertainment, games, access to business systems away from work—even a personal ATM that lets you download funds into a smart card via your phone.
The Java platform's device-independent nature and network-oriented design ensures that services such as these can function securely on many different consumer platforms.
You don't need to be a mechanic to drive a car. Why should you have to be a “system administrator” to use a computer?
With Java software, you don't have to be one. Java technology eliminates many of the problems associated with installing and running applications. That's because generally the Java user does not have to configure, load, or install anything. Instead, computing devices tap into the network and funnel its power to the user. Upgrades are automatic, making installation and configuration obsolete.
It's a whole new way of thinking about computers. Just click on a link or press a button and you’re ready to go.
Most important, right from the beginning, the Java platform was designed to run programs securely on networks, which means that it integrates safely with the existing systems on your network.
Java technology-based software is typically delivered over a network and can also be installed on computers from traditional media such as CD-ROMs. The same program or software component can run on a variety of computers and devices.
Programs written in the Java programming language run on so many different kinds of systems thanks to a component of the platform called the Java virtual machine—a kind of translator that turns general Java platform instructions into tailored commands that make the devices do their work.
Understanding the World Wide Web
The World Wide Web is a system of Internet servers that supports hypertext to access several Internet protocols on a single interface. The World Wide Web is often abbreviated as the Web or WWW.
The World Wide Web was developed in 1989 by Tim Berners-Lee of the European Particle Physics Lab (CERN) in Switzerland. The initial purpose of the Web was to use networked hypertext to facilitate communication among its members. who were located in several countries. Word was soon spread beyond CERN, and a rapid growth in the number of both developers and users ensued. In addition to hypertext, the Web began to incorporate graphics, video and sound. The use of the Web has now reached global proportions.
Almost every protocol type available on the Internet is accessible on the Web. Internet protocols are sets of rules that allow for intermachine communication on the Internet. The following major protocols are accessible on the web.
· e-mail (Simple Mail Transport Protocol): Distributes electronic messages and files to one or more electronic mailboxes.
· Telnet (Telnet Protocol): Facilitates login to a computer host to execute commands.
· FTP (File Transfer Protocol): Transfers text or binary files between an FTP server and client.
· Usenet (Network News Transfer Protocol): Distributes Usenet news articles derived from topical discussions on newsgroups.
· HTTP (Hypertext Transfer Protocol): Transmits hypertext over networks. This is the protocol of the WWW.
Many other protocols are available on the Web. To name just one example, the Voice over Internet Protocol (VoIP) allows users to place a telephone call over the Web.
The World Wide Web provides a single interface for accessing all these protocols. This creates a convenient and user-friendly environment. It is no longer necessary to be conversant in these protocols within separate command-level environments. The Web gathers together these protocols into a single system. Because of this feature and because of the Web's ability to work with multimedia and advanced programming languages, the World Wide Web is the fastest-
growing component of the Internet.
The operation of the Web relies primarily on hypertext as its means of information retrieval. Hypertext is a document containing words that connect to other documents. These words are called links and are selectable by the user. A single hypertext document can contain links to many documents. In the context of the Web, words or graphics may serve as links to other documents, images, video and sound. Links may or may not follow a logical path, as each connection is programmed by the creator of the source document. Overall, the WWW contains a complex virtual web of connections among a vast number of documents, graphics, videos and sounds.
Producing hypertext for the Web is accomplished by creating documents with a language called Hypertext Markup Language, or HTML. With HTML, tags are placed within the text to accomplish document formatting, visual features such as font size, italics and bold, and the creation of hypertext links. Graphics may also be incorporated into an HTML document. HTML is an evolving language, with new tags being added as each upgrade of the language is developed and released. The World Wide Web Consortium, led by Tim Berners-Lee, coordinates the efforts of standardizing HTML.
The World Wide Web consists of files called pages or Web pages, containing information and links to resources throughout the Internet.
Web pages can be created by user activity. For example, if you visit a Web search engine and enter keywords on the topic of your choice, a page will be created containing the results of your search. In fact, an increasing amount of information found on the Web today is served from databases, creating temporary Web pages “on the fly” in response to user queries.
Access to Web pages may be accomplished by:
· Entering an Internet address and retrieving a page directly.
· Browsing through pages and selecting links to move from one page to another.
· Searching through subject directories linked to organized collections of Web pages.
· Entering a search statement at a search engine to retrieve pages on the topic of your choice.
URL stands for Uniform Resource Locator. The URL specifies the Internet address of a file stored on a host computer connected to the Internet. Every file on the Internet, no matter what its access protocol, has a unique URL. Web software programs use the URL to retrieve the file from the host computer and the directory in which it resides. This file is then displayed on the monitor connected to the user's local machine.
URLs are translated into numeric addresses using the Internet Domain Name System (DNS). The numeric address is actually the “real” URL. Since numeric strings are difficult for humans to use, alphanumeric addresses are employed by end users. Once the translation is made, the Web server can send the requested page to the user's Web browser.
This is the format of the URL:
protocol: //host/path/filename
For example, this is a URL on the home page of the House Committee on Agriculture of the U.S. House of Representatives:
http: //www.house.gov/agriculture/schedule.htm
This URL is typical of addresses hosted in domains in the United States.
Structure of this URL.
· Protocol: http.
· Host computer name: WWW.
· Second-level domain name: house.
· Top-level domain name: gov.
· Directory name: agriculture.
· File name: schedule. htm.
Note how much information about the content of the file is present in this well-constructed URL. Other examples:
telnet://opac.Albany.edu the University at Albany library text-based catalog.
ftp://ftp.HU.net/graphics/picasso a file at an FTP site.
Several top-level domains (TLDs) are common in the United States, as shown in Table 3-4.
Table 3-4 Several top-level domains
|
Domain |
Meaning |
Domain |
Meaning |
|
com |
commercial enterprise |
mil |
U. S.military entity |
|
edu |
educafionai institution |
net |
network access provider |
|
gov |
U. S.government entity |
org |
usually nonprofit organizations |
New domain names were approved in November 2000 by the Internet Corporation for Assigned Names and Numbers(ICANN): .biz, .museum, .info, .pro (for professionals). Name (for individuals), .aero(for the aerospace industry), and .coop(for cooperatives). These domain names are beginning to become available.
In addition, dozens of domain names have been assigned to identify and locate files stored on host computers in countries around the world. These are referred to as two-letter Internet country codes, and have been standardized by the International Standards Organization as ISO 3166.
To access the World Wide Web, you must use a Web browser. A browser is a software program that allows users to access and navigate the World Wide Web. There are two types of browsers:
（1）Graphical. Text, images, audio and video are retrievable through a graphical software program such as Netscape Navigator and Internet Explorer. These browsers are available for both Windows-based and Macintosh computers. Navigation is accomplished by pointing and clicking with a mouse on highlighted words and graphics.
You can install a graphical browser such as Netscape Navigator in your Windows-based or Macintosh machine. Navigator is available for downloading on the Netscape Web site: http:// home.netscape.com/. Microsoft's Internet Explorer is available from the Microsoft Web site: http://www.microsoft.com/. To use these programs to access the Web, you need a connection to the Internet. This is accomplished through such means as an Ethernet connection, a dialup connection known as a SLPP or PPP, or a cable modem.
（2）Text. Lynx is a browser that provides access to the Web in text. only mode. Navigation is accomplished by highlighting emphasized words in the screen with the arrow up and down keys, and then pressing the forward arrow (or Enter) key to follow the link.
· Extending the Browser: Plug-ins.
Software programs may be configured to a Web browser in order to enhance its capabilities. When the browser encounters a sound, image or video file, it hands off the data to other programs, called plug-ins, to run or display the file. Working in conjunction with plug-ins, browsers can offer a seamless multimedia expedience. Many plug-ins are available for free.
File formats requiring plug-ins are known as MIME types. MIME stands for Multimedia Internet Mail Extension, and was originally developed to help e-mail software handle a variety of binary (non-ASCII) file attachments. The use of MIME has expanded to the Web. For example, the basic MIME type handled by Web browsers is text/html associated with the file extension. html.
A common plug-in utilized on the Web is the Adobe Acrobat Reader. The Acrobat Reader allows you to view documents created in Adobe's Portable Document Format (PDF). These documents are the MIME type application/pdf and are associated with the file extension .pdf. When the Acrobat Reader has been configured to your browser, the program will open and display the file requested when you click on a hyper-linked file name with the suffix .pdf. The latest versions of the Acrobat Reader allow for the viewing of documents within the browser window.
Web browsers are often standardized with a small suite of plug-ins, especially for playing multimedia content. Additional plug-ins may be obtained at the browser's Web site, at special download sites on the Web, or from the Web sites of the companies that created the programs. The number of available plug-ins is increasing rapidly.
Once a plug-in is configured to your browser, it will automatically launch when you choose to access a file type that it uses.
· Beyond Plug-ins: ActiveX.
ActiveX is a technology developed by Microsoft which may make plug-ins less necessary. ActiveX offers the opportunity to embed animated objects, data and computer code on Web pages. A web browser supporting ActiveX can render most items encountered on a Web page. For example, ActiveX allows users to view three-dimensional VRML worlds in a Web browser without the use of a VRML plug-in. As another example of the power of ActiveX, this techn-
ology can allow you to view and edit PowerPoint presentations directly within your Web browser. ActiveX works best with Microsoft's Internet Explorer Browser.
Today's World Wide Web presents an ever-diversified experience of multimedia, program-
ming languages and real-time communication. There is no question that it is a challenge to keep up with the rapid pace of developments. The following presents a brief description of some of the more important trends to watch.
（1）Multimedia.
The Web has become a broadcast medium. It is possible to listen to audio and video over the Web both pre-recorded and live. For example, you can visit the sites of various news organizations and view the same videos shown on the nightly television news. Several plug-ins are available for viewing these videos. For example, Apple's Quick Time Player downloads files with the .mov extension and displays these as “movies” in a small window on your computer screen. Quick Time files can be quite large, and it may take patience to wait for the entire movie to download into your computer before you can view it.
The problem of slow download times has been answered by a revolutionary development in multimedia capability: streaming media. In this case, audio or video files are played as they are downloading or streaming into your computer. Only a small wait, called buffering, is necessary before the file begins to play. The RealPlayer plug-in plays streaming audio and video files. Extensive files such as interviews, speeches and hearings work very well with the RealPlayer. The RealPlayer is also ideal for the broadcast of real-time events. These may include press conferences, live radio and television broadcasts, concerts, etc. The Windows Media Player is another streaming media player. Many sites offer the option to use one player or the other. A list of sites that make use of these programs is available on the page, multimedia on the Web.
Shockwave presents another multimedia experience. Shockwave allows for the creation and implementation of an entire multimedia display combining graphics, animation and sound.
Sound files, including music, may also be heard on the Web. It is not uncommon to visit a Web page and hear background music. Sound files are also available for downloading independent of Web page visits. Sound files of many types are supported by the Web with the appropriate plug-ins. The MP3 file format, and the choice of supporting plug-ins, is the latest music trend to sweep the Web. The famous Napster site allows for the exchange of MP3 files.
Live cams are another aspect of the multimedia experience available on the Web. Live cams are video cameras that send their data in real-time to a Web server. These cams may appear in all kinds of locations, both serious and whimsical: an office, on top of a building, a scenic locale, a special event, and so on.
（2）Programming languages and functions.
The use of existing and new programming languages has extended the capabilities of the Web. What follows is a basic guide to a group of the more common languages and functions in use on the Web today.
· CGI:CGI (Common Gateway Interface) refers to a specification by which programs can communicate with a Web server. A CGI program or script, is any program designed to accept and return data that conforms to the CGI specification. The program can be written in any programming language, including C, Perl, and Visual Basic Script. A common use for a CGI script is to process an interactive form on a Web page. For example, you might fill out a form ordering a book through Interlibrary Loan. The script processes your information and sends it to a designated e-mail address in the Interlibrary Loan department.
· ASP (Active Server Pages):Another type of dynamically generated Web page is called Active Server Pages (ASP). Developed by Microsoft, ASPs are HTML pages that include scripting and create interactive Web server applications. The scripts run on the server, rather than on the Web browser, to generate the HTML pages sent to browsers. Visual Basic and JScript (a subset of JavaScript) are often used for the scripting. ASPs end in the file extension .asp.
· Java/Java Applets:Java is probably the most famous of the programming languages of the Web. Java is an object-oriented programming language similar to C++. Developed by Sun Microsystems, the aim of Java is to create programs that will be platform independent. The Java motto is, “Write once, run anywhere”. A perfect Java program should work equally well on a PC, Macintosh, UNIX, and so on, without any additional programming. This goal has yet to be realized. Java can be used to write applications for both Web and non-Web use.
Web-based Java applications are usually in the form of Java applets. These are small Java programs called from an HTML page that can be downloaded from a Web server and run on a Java—compatible Web browser. A few examples include live newsfeeds, moving images with sound, calculators, charts and spreadsheets, and interactive visual displays. Java applets can tend to load slowly, but programming improvements should lead to a shortened loading time.
· Java Script/JScript:JavaScript is a programming language created by Netscape Communications. Small programs written in this language are embedded within an HTML page, or called externally from the page, to enhance the page's the functionality. Examples of JavaScript include moving tickers, drop-down menus, real-time calendars and clocks, and mouse-over interactions. JScript is a similar language developed by Microsoft and works with the company's Internet Explorer browser.
· VRMI:VRMI (Virtual Reality Modeling Language) allows for the creation of three- dimensional worlds. These may be linked from Web pages and displayed with a VRML, viewer. Netscape Communicator comes with the Cosmo viewer for experiencing these three-dimensional worlds. One of the most interesting aspects of VRML is the option to “enter” the world and control your movements within the world.
· XML:XML (Extensible Markup Language) is a Web page creation language that enables designers to create their own customized tags to provide functionality not available with HTML. XML is a language of data structure and exchange, and allows developers to separate form from content. At present, this language is little used as Web browsers are only beginning to support it. In May 1999, however, the W3 Consortium announced that HTML 4.0 has been recast as an XML application called XHTML. This move will have a significant impact on the future of both XML and HTML.
（3）Real-Time Communication.
Text, audio and video communication call occur in real-time on the Web. This capability allows people to conference and collaborate in real-time. In general, the faster the Internet connection, the more successful the experience.
At its simplest, chat programs allow multiple users to type to each other in real-time. Internet Relay Chat and America Online's Instant Messenger are prime examples of this type of program. The development of a messaging protocol is underway. Such a protocol would allow for the expansion of this capability throughout the Internet.
More enhanced real-time communication offers an audio and/or video component. CU-See Me is one of the most popular software programs of this type. Even more elaborate are programs that allow for true real-time collaboration. Microsoft's NetMeeting and Netscape's Conference (available with Communicator) are good examples of this.
Featured collaboration tools include seven programs.
· audio:conduct a telephone conversation on the Web.
· video:view your audience.
· file transfer:send files back and forth among participants.
· chat:type in real-time.
· whiteboard:draw, mark up, and save images on a shared window or board.
· document/application sharing:view and use a program on another’s desktop machine.
· collaborative Web browsing:visit Web pages together.
Currently no standard exists that will work among all conferencing programs.
· Push:Push refers to a technology that sends data to a program without the program's request. This is the opposite of the typical “pull” of the Web, in which the user clicks on a link to request a file from a server. With push, the data is sent automatically. Content is sent through a “channel”. The early Web-based implementation of push was commercial. Push can also be used to deliver software upgrades to a desktop machine.