1.2 The Network Edge

In the previous section we presented a high-level overview of the Internet and networking
protocols. We are now going to delve a bit more deeply into the components
of a computer network (and the Internet, in particular). We begin in this
section at the edge of a network and look at the components with which we are most
familiar-namely, the computers, PDAs, cell phones and other devices that we use
on a daily basi s. In the next section we'll move from the network edge to the network
core and examine switching and routing in computer networks.

Recall from the previous section that in computer networking jargon, the computers
and other devices connected to the Internet are often referred to as end systems.
They are referred to as end systems because they sit at the edge of the Internet,
as shown in Figure 1.3. The Internet's end systems include desktop computers (e.g., desktop pes, Macs, and Linux boxes), servers (e.g., Web and e-mail servers), and
mobile computers (e.g., portable computers, PDAs. and phones with wireless Internet
connections). Furthermore, an increasing number of alternative devices are being attached to the Internet as end systems (see sidebar). E11d systems are also referred to as hosts because they host (that i , run) applisometim
cation programs such as a Web browser program, a Web server program, an e-mail reader program, or an e-mail server program. Throughout this book we will use the
terms hos ts and end systems interchangeably; that is, host =end system. Hosts are
sometimes further di vided into two categories: clients and servers. Informall y,
clients tend to be desktop and mobile pes, PDAs, and so on, whereas servers tend to be more powerful machines that store and distribute Web pages, stream video,
relay e-mail, and so on.