Java Reference
In-Depth Information
One of the biggest secrets about Java is that it makes writing network programs easy. In
fact, it is far easier to write network programs in Java than in almost any other language.
This topic shows you dozens of complete programs that take advantage of the Internet.
Some are simple textbook examples, while others are completely functional applica‐
tions. One thing you'll notice in the fully functional applications is just how little code
is devoted to networking. Even in network-intensive programs like web servers and
clients, almost all the code handles data manipulation or the user interface. The part of
the program that deals with the network is almost always the shortest and simplest. In
brief, it is easy for Java applications to send and receive data across the Internet.
This chapter covers the background networking concepts you need to understand be‐
fore writing networked programs in Java (or, for that matter, in any language). Moving
from the most general to the most specific, it explains what you need to know about
networks in general, IP and TCP/IP-based networks in particular, and the Internet. This
chapter doesn't try to teach you how to wire a network or configure a router, but you
will learn what you need to know to write applications that communicate across the
Internet. Topics covered in this chapter include the nature of networks; the TCP/IP layer
model; the IP, TCP, and UDP protocols; firewalls and proxy servers; the Internet; and
the Internet standardization process. Experienced network gurus may safely skip this
chapter, and move on to the next chapter where you begin developing the tools needed
to write your own network programs in Java.
Networks
A
network
is a collection of computers and other devices that can send data to and
receive data from one another, more or less in real time. A network is often connected
by wires, and the bits of data are turned into electromagnetic waves that move through
the wires. However, wireless networks transmit data using radio waves; and most long-
distance transmissions are now carried over fiber-optic cables that send light waves
through glass filaments. There's nothing sacred about any particular physical medium
for the transmission of data. Theoretically, data could be transmitted by coal-powered
computers that send smoke signals to one another. The response time (and environ‐
mental impact) of such a network would be rather poor.
Each machine on a network is called a
node
. Most nodes are computers, but printers,
routers, bridges, gateways, dumb terminals, and Coca-Cola™ machines can also be no‐
des. You might use Java to interface with a Coke machine, but otherwise you'll mostly
talk to other computers. Nodes that are fully functional computers are also called
hosts
. I will use the word
node
to refer to any device on the network, and the word
host
to refer to a node that is a general-purpose computer.
Every network node has an
address
, a sequence of bytes that uniquely identifies it. You
can think of this group of bytes as a number, but in general the number of bytes in an
address or the ordering of those bytes (big endian or little endian) is not guaranteed to
