Part I: Networking
Fundamentals
Chapter 1 Introduction to Computer Networking Concepts
Chapter 2 The TCP/IP and OSI Networking Models
Chapter 3 Fundamentals of LANs
Chapter 4 Fundamentals of WANs
Chapter 5 Fundamentals of IP Addressing and Routing
Chapter 6 Fundamentals of TCP/IP Transport, Applications, and Security
C H A P T E R1
Introduction to Computer
Networking Concepts
This chapter gives you a light-hearted perspective about networks, how they were originally
created, and why networks work the way they do. Although no specific fact from this
chapter happens to be on any of the CCNA exams, this chapter helps you prepare for the
depth of topics you will start to read about in Chapter 2, “The TCP/IP and OSI Networking
Models.” If you are brand new to networking, this short introductory chapter will help you
get ready for the details to follow. If you already understand some of the basics of TCP/IP,
Ethernet, switches, routers, IP addressing, and the like, go ahead and skip on to Chapter 2.
The rest of you will probably want to read through this short introductory chapter before
diving into the details.
Perspectives on Networking
So, you are new to networking. You might have seen or heard about different topics relating
to networking, but you are only just now getting serious about learning the details. Like
many people, your perspective about networks might be that of a user of the network, as
opposed to the network engineer who builds networks. For some, your view of networking
might be based on how you use the Internet, from home, using a high-speed Internet
connection. Others of you might use a computer at a job or at school, again connecting to
the Internet; that computer is typically connected to a network via some cable. Figure 1-1
shows both perspectives of networking.
Figure 1-1 End-User Perspective on Networks
The top part of the figure shows a typical high-speed cable Internet user. The PC connects
to a cable modem using an Ethernet cable. The cable modem then connects to a cable TV
(CATV) outlet on the wall using a round coaxial cable—the same kind of cable used to
connect your TV to the CATV wall outlet. Because cable Internet services provide service
Home User
PC with
Ethernet Card
Office User
PC with
Ethernet Card
The Internet
Ethernet
Cable
CATV
Cable
Ethernet Cable
6 Chapter 1: Introduction to Computer Networking Concepts
continuously, the user can just sit down at the PC and start sending e-mail, browsing
websites, making Internet phone calls, and using other tools and applications as well.
Similarly, an employee of a company or a student at a university views the world as a
connection through a wall plug. Typically, this connection uses a type of local-area network
(LAN) called Ethernet. Instead of needing a cable modem, the PC connects directly to an
Ethernet-style socket in a wall plate (the socket is much like the typical socket used for
telephone cabling today, but the connector is a little larger). As with high-speed cable Internet
connections, the Ethernet connection does not require the PC user to do anything first to
connect to the network—it is always there waiting to be used, similar to the power outlet.
From the end-user perspective, whether at home, at work, or at school, what happens
behind the wall plug is magic. Just as most people do not really understand how cars work,
how TVs work, and so on, most people who use networks do not understand how they work.
Nor do they want to! But if you have read this much into Chapter 1, you obviously have a
little more interest in networking than a typical end user. By the end of this book, you
will have a pretty thorough understanding of what is behind that wall plug in both cases
shown in Figure 1-1.
The CCNA exams, and particularly the ICND1 (640-822) exam, focus on two major
branches of networking concepts, protocols, and devices. One of these two major branches
is called enterprise networking. An enterprise network is a network created by one
corporation, or enterprise, for the purpose of allowing its employees to communicate. For
example, Figure 1-2 shows the same type of PC end-user shown in Figure 1-1, who is now
communicating with a web server through the enterprise network (represented by a cloud)
created by Enterprise #2. The end-user PC can communicate with the web server to do
something useful for the company—for instance, the user might be on the phone with a
customer, with the user typing in the customer’s new order in the ordering system that
resides in the web server.
Figure 1-2 An Example Representation of an Enterprise Network
Office User
PC with
Ethernet Card
Ethernet Cable
Web Server
Enterprise #2
Perspectives on Networking 7
The second major branch of networking covered on the ICND1 exam is called small office/
home office, or SOHO. This branch of networking uses the same concepts, protocols, and
devices used to create enterprise networks, plus some additional features that are not
needed for enterprises. SOHO networking allows a user to connect to the Internet using
a PC and any Internet connection, such as the high-speed cable Internet connection shown
in Figure 1-1. Because most enterprise networks also connect to the Internet, the SOHO
user can sit at home, or in a small office, and communicate with servers at the enterprise
network, as well as with other hosts in the Internet. Figure 1-3 shows the concept.
Figure 1-3 SOHO User Connecting to the Internet and Other Enterprise Networks
The Internet itself consists of most every enterprise network in the world, plus billions of
devices connecting to the Internet directly through Internet service providers (ISPs). In fact,
the term itself—Internet—is formed by shortening the phrase “interconnected networks.”
To create the Internet, ISPs offer Internet access, typically using either a cable TV line, a
phone line using digital subscriber line (DSL) technology, or a telephone line with a modem.
Each enterprise typically connects to at least one ISP, using permanent connections
generally called wide-area network (WAN) links. Finally, the ISPs of the world also
connect to each other. These interconnected networks—from the smallest single-PC home
network, to cell phones and MP3 players, to enterprise networks with thousands of
devices—all connect to the global Internet.
NOTE In networking diagrams, a cloud represents a part of a network whose details
are not important to the purpose of the diagram. In this case, Figure 1-2 ignores the
details of how to create an enterprise network.
Home User
PC with
Ethernet Card
Office User
PC with
Ethernet Card
The Internet
-
Many ISPs
Ethernet
Cable
Ethernet
Cable Enterprise #2
Web Server
Enterprise #1
Enterprise #3
Web Server
Web Server
CATV
Cable
PC
PC