Linux with Operating System Concepts

loopback device
, also 
known as your local network or local host. Software will sometimes communicate 
through this device as if the device was sending messages out onto the network, but 
lo does not actually reach a network. Therefore, you need at least one other interface 
which can take on many different forms. The most common interface is an Ethernet 
device. This might be an Ethernet card to connect your computer to a LAN, which is 
a wired connection, or a wireless card. The Ethernet device, in Linux, is denoted with 
the name eth. The actual name will be 
eth
#
where 
#
is a digit, for instance eth0. It is 
possible that your computer will have multiple Ethernet devices, in which case, the 
numbers will differ, for instance, by having eth0 and eth1. Other types of interfaces 

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Linux with Operating System Concepts
include ppp (point-to-point interface), tr (token ring interface), slip or plip (serial line 
Internet protocol, parallel line Internet protocol), and sit (simple Internet transitions).
• Run your network services. There are numerous services related to the network. The 
most important is 
network
. Others include the firewall service, interface device 
services, network authentication services, and network file system services. If you are 
uninterested in the services offered, these others do not need to be started (although 
it is important to always run the firewall, we will discuss the firewall in Section 12.6). 
The network service should automatically be started in run level 3 or 5; so, you should 
not have to worry about it. We discuss some of these services in Section 12.3.
• Have access to a network-broadcast device. In a network, this would be a physical 
connection from your computer’s network interface device to a hub, switch, or router 
(most likely a network switch or router). From a home computer, you will probably 
connect via a wireless card to a MODEM in your household. This wireless connec-
tion is similar to your networked computer connecting to a router. In your case, the 
MODEM will serve as a router within your household. The modern MODEMs permit 
multiple devices to connect to them in case you want to develop your own home net-
work. Your MODEM then connects to an Internet service provider via the telephone 
line, coaxial cable, or fiber-optic cable.
In this chapter, we will explore the services, configuration, and data files available in 
Linux to configure your computer to communicate with the network. We will assume 
that you will be communicating not just with a local area network but with the Internet 
through the TCP/IP protocol.
12.2 COMPUTER NETWORKS AND TCP/IP
12.2.1 Broadcast Devices
Before we begin our examination of Linux network configuration, let us first discuss com-
puter networks. A computer network is a collection of computers and other devices con-
nected together through some medium, for instance, twisted-wire pair or fiber-optic cable. 
Aside from computers, printers, servers, and so forth, we also need network-broadcast 
devices. These devices fall into four categories: hubs, switches, routers, and gateways. The 
intention of these devices is to permit computers to communicate with each other without 
having to directly connect each computer to every other computer.
In the hub, the devices within some small network (a subnet) are connected to one hub. 
Any message that reaches the hub is then broadcast to all connected devices. The switch is a 
more capable device. Like the hub, computers in a subnet connect to it. But the subnet retains 
device addresses so that any incoming message is directed to the device that matches the 
message’s destination address. These addresses are not IP addresses but instead addresses 
for the lowest layer of the network (the link layer), for instance MAC (media access control) 
addresses. The router connects together networks; so, it is sort of like a switch for switches. 
Messages that arrive at a router are then directed to the proper network or subnet. At this 

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Network Configuration
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level, addresses are network addresses, for instance, IPv4 addresses. Finally, gateways are 
used to connect one network to another type of network. You may find a gateway to be 
used as an organization’s point of presence onto the Internet.
Figure 12.1 illustrates the layout of a local area network. In this network, we find two 
subnetworks, each with their own broadcast switch. Connected to one switch are four 
desktop computers and a printer. Connected to the other switch are three desktop com-
puters and a printer. The two switches are connected to a router. Each of the resources con-
nects to its broadcast device via a twisted-wire pair cable. Not shown here is the connection 
of the router to other networks.
12.2.2 The TCP/IP Protocol Stack
For Internet communication, all devices must utilize the TCP/IP protocol. This four-level 
protocol is actually a suite of protocols in which each layer can be implemented by one of 
the many different specific protocols. What TCP/IP provides is the rules for how commu-
nication can take place between resources on the Internet. These rules include how mes-
sages are broken into packets, how addressing and error-handling information is added to 
the packets, how the packets are treated as they move from location to location, how two-
way communication is established, and how received packets are pieced together to make 
a message. Figure 12.2 demonstrates the four layers of the TCP/IP protocol. We briefly 
examine each layer below.
First, application software takes your message and produces the initial, application- neutral 
message to be transmitted. The application software utilizes one of the many protocols 
Pc
Pc
Pc
Pc
Pc
Twisted-pair cable
Router
Crossover cable
Switch
Switch
Twisted-pair cable
Twisted-pair cable
Twisted-pair cable
Twisted-pair cable
Printer
Printer
Pc
Pc
Twisted-pair cable
Twisted-pair cable
FIGURE 12.1 
A local area network. (Adapted from Fox, R.