Section 1.7
The Internet and the World-Wide Web
COMPUTERS CAN BE CONNECTED together
on networks. A computer
on a network can communicate with other computers on the
same network by exchanging data and files or by sending
and receiving messages. Computers on a network can even
work together on a large computation.
Today, millions of computers throughout the world are
connected to a single huge network called the
Internet. New computers are being
connected to the Internet every day. In fact, a computer
can join the Internet temporarily by using a modem to establish
a connection through telephone lines.
There are elaborate protocols
for communication over the Internet. A protocol is simply a
detailed specification of how communication is to proceed.
For two computers to communicate at all, they must both be using
the same protocols. The most basic protocols on the Internet are the
Internet Protocol (IP),
which specifies how data is to be physically transmitted from one
computer to another, and the Transmission
Control Protocol (TCP), which ensures that data sent
using IP is received in its entirety and without error.
These two protocols, which are referred to collectively
as TCP/IP, provide a foundation for communication. Other
protocols use TCP/IP to send specific types of information
such as files and electronic mail.
All communication over the Internet is in the form of
packets. A packet
consists of some data being sent from one computer to another,
along with addressing information that
indicates where on the Internet that data is supposed to go.
Think of a packet as an envelope with an address on the outside
and a message on the inside. (The message is the data.)
The packet also includes a "return
address," that is, the address of the sender. A packet can
hold only a limited amount of data; longer messages must be divided
among several packets, which are then sent individually over the net
and reassembled at their destination.
Every computer on the Internet has an IP
address, a number that identifies it uniquely among all
the computers on the net. The IP address is used for addressing
packets. A computer can only send data to another computer on the
Internet if it knows that computer's IP address. Since people
prefer to use names rather than numbers, many computers are
also identified by names, called domain
names. For example, the main computer at Hobart and William
Smith Colleges has the domain name hws3.hws.edu. (Domain names
are just for convenience; your computer still needs to know
IP addresses before it can communicate. There are computers
on the Internet whose job it is to translate domain names to
IP addresses. When you use a domain name, your computer sends
a message to a domain name server to find out the corresponding
IP address. Then, your computer uses the IP address, rather than
the domain name, to communicate with the other computer.)
The Internet provides a number of services to the computers
connected to it (and, of course, to the users of those computers).
These services use TCP/IP to send various types of data over the
net. Among the most popular services are remote login, electronic
mail, FTP, and the World-Wide Web.
Remote login allows a person using
one computer to log on to another computer. (Of course,
that person needs to know a user name and password for an
account on the other computer.) There are several different
protocols for remote login, including the traditional
telnet and the more secure
ssh (secure shell).
Telnet and ssh provide only a
command-line interface. Essentially, the first
computer acts as a terminal for the second. Remote login is often
used by people who are away from home to access their
computer accounts back home -- and they can do so from any
computer on the Internet, anywhere in the world.
Electronic mail, or email, provides
person-to-person communication over the Internet. An email message
is sent by a particular user of one computer to a particular
user of another computer. Each person is identified by a
unique email address, which consists of the domain name of
the computer where they receive their mail together with their
user name or personal name. The email address has the form
"[email protected]". For example, my own email
address is: [email protected]. Email is actually transferred from
one computer to another using a protocol called SMTP (Simple
Mail Transfer Protocol). Email might still be the most common and
important use of the Internet, although it has certainly been
challenged in popularity by the World-Wide Web.
FTP (File Transport Protocol)
is designed to copy files from one computer to another. As with
remote login, an FTP user needs a user name and password to get access
to a computer. However, many computers have been set up with
special accounts that can be accessed through FTP with the
user name "anonymous" and any password. This so-called
anonymous FTP can be used to make
files on one computer publically available to anyone with
Internet access.
The World-Wide Web (WWW)
is based on pages which can contain information
of many different kinds as well as links
to other pages. These pages are viewed with a Web browser
program such as Netscape or Internet Explorer.
Many people seem to think that the World-Wide Web is
the Internet, but it's really just a graphical user interface to the Internet.
The pages that you view with a Web browser are just files that are stored
on computers connected to the Internet. When you tell your Web browser
to load a page, it contacts the computer on which the page is stored
and transfers it to your computer using a protocol known as
HTTP (HyperText Transfer Protocol).
Any computer on the Internet can publish pages on the World-Wide
Web. When you use a Web browser, you have access to
a huge sea of interlinked information that can be navigated
with no special computer expertise. The Web is the most
exciting part of the Internet and is driving the Internet to
a truly phenomenal rate of growth. If it fulfills its promise,
the Web might become a universal and fundamental part of
everyday life.
I should note that a typical Web browser can use other protocols
besides HTTP. For example, it can also use FTP to transfer files.
The traditional user interface for FTP was a command-line interface,
so among all the other things it does, a Web browser provides a
modern graphical user interface for FTP. This allows people to
use FTP without even knowing that there is such a thing!
(This fact should help you understand the difference between a
program and a protocol.
FTP is not a program. It is a protocol, that is, a set of standards for
a certain type of communication between computers. To use FTP,
you need a program that implements those standards. Different FTP
programs can present you with very different user interfaces.
Similarly, different Web browser programs can present very different
interfaces to the user, but they must all use HTTP to get information
from the Web.)
Now just what, you might be thinking, does all this have
to do with Java? In fact, Java is intimately associated with
the Internet and the World-Wide Web. As you have seen in the
previous section, special Java programs called applets are
meant to be transmitted over the Internet and displayed on
Web pages. A Web server transmits a Java applet just as
it would transmit any other type of information. A Web browser
that understands Java -- that is, that includes an interpreter
for the Java virtual machine -- can then run the applet right
on the Web page. Since applets are programs, they can do
almost anything, including complex interaction with the
user. With Java, a Web page becomes more than just a passive
display of information. It becomes anything that programmers
can imagine and implement.
Its association with the Web is not Java's only advantage.
But many good programming
languages have been invented only to be soon forgotten. Java has
had the good luck to ride on the coattails of the Web's immense
and increasing popularity.
End of Chapter 1
