Chapter 1 Introduction to Networking

TCP/IP traces its origins to a research project funded by the United States Defense
Advanced Research Projects Agency (DARPA) in 1969. The ARPANET was an
experimental network that was converted into an operational one in 1975 after it
had proven to be a success.
In 1983, the new protocol suite TCP/IP was adopted as a standard, and all hosts on
the network were required to use it. When ARPANET finally grew into the
Internet (with ARPANET itself passing out of existence in 1990), the use of
TCP/IP had spread to networks beyond the Internet itself. Many companies have
now built corporate TCP/IP networks, and the Internet has grown to a point at
which it could almost be considered a mainstream consumer technology. It is
difficult to read a newspaper or magazine now without seeing reference to the
Internet; almost everyone can now use it.
For something concrete to look at as we discuss TCP/IP throughout the following
sections, we will consider Groucho Marx University (GMU), situated somewhere
in Fredland, as an example. Most departments run their own Local Area Networks,
while some share one and others run several of them. They are all interconnected
and hooked to the Internet through a single high-speed link.
Suppose your Linux box is connected to a LAN of Unix hosts at the Mathematics
department, and its name is erdos. To access a host at the Physics department, say
quark, you enter the following command:
$ rlogin quark.physics
Welcome to the Physics Department at GMU
(ttyq2) login:
At the prompt, you enter your login name, say andres, and your password. You are
then given a shell[2] on quark, to which you can type as if you were sitting at the
system's console. After you exit the shell, you are returned to your own machine's
prompt. You have just used one of the instantaneous, interactive applications that
TCP/IP provides: remote login.
[2] The shell is a command-line interface to the Unix operating
system. It's similar to the DOS prompt in a Microsoft Windows
environment, albeit much more powerful.
While being logged into quark, you might also want to run a graphical user
interface application, like a word processing program, a graphics drawing
program, or even a World Wide Web browser. The X windows system is a fully
network-aware graphical user environment, and it is available for many different
computing systems. To tell this application that you want to have its windows
displayed on your host's screen, you have to set the DISPLAYenvironment
variable:
$ DISPLAY=erdos.maths:0.0
$ export DISPLAY
If you now start your application, it will contact your X server instead of quark's,
and display all its windows on your screen. Of course, this requires that you have
X11 runnning on erdos. The point here is that TCP/IP allows quarkand erdosto
send X11 packets back and forth to give you the illusion that you're on a single
system. The network is almost transparent here.
Another very important application in TCP/IP networks is NFS, which stands for
Network File System. It is another form of making the network transparent,
because it basically allows you to treat directory hierarchies from other hosts as if
they were local file systems and look like any other directories on your host. For
example, all users' home directories can be kept on a central server machine from
which all other hosts on the LAN mount them. The effect is that users can log in to
any machine and find themselves in the same home directory. Similarly, it is
possible to share large amounts of data (such as a database, documentation or
application programs) among many hosts by maintaining one copy of the data on a
server and allowing other hosts to access it. We will come back to NFS in Chapter
14, The Network File System.
Of course, these are only examples of what you can do with TCP/IP networks. The
possibilities are almost limitless, and we'll introduce you to more as you read on
through the book.
We will now have a closer look at the way TCP/IP works. This information will
help you understand how and why you have to configure your machine. We will
start by examining the hardware, and slowly work our way up