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Chapter 9
Setting Up Internet Sharing
In This Chapter
Using an Internet gateway or router
Obtaining an IP address automatically in Windows 9x/2000/XP and Mac OS 9/X
Internet connection sharing in Windows 98/98 SE/2000/Me/XP and Mac OS X




O ne of the most popular uses of personal computers is to access the
Internet. In this chapter, we describe how you can use a network,
including a wireless network, to share a single Internet connection among all
the computers on the network. We also describe how to obtain an Internet
Protocol (IP) address automatically in Windows 9x/2000/XP and Mac OS 9/X.
In addition, the chapter explains how to set up sharing of Internet connec-
tions without the need to buy a router in Windows 98/98 SE/2000/Me/XP and
Mac OS X.

In Chapter 7, we describe how to set up and configure wireless network inter-
face adapters using the installation software that accompanies the adapters.
When you set up wireless adapters that way, the installation software (in
most cases) properly configures the adapter to make it possible for comput-
ers on the network to communicate and to take advantage of the Internet-
sharing capabilities of Internet gateways, Dynamic Host Configuration
Protocol (DHCP) servers, and cable/digital subscriber line (DSL) routers.
Occasionally, however, you might need to change network settings of a wire-
less network adapter.




Deciding How to Share Your
Internet Connection
Whether you™ve installed a wireless network or are using some other type of
network devices to create a home network, you no doubt want all your net-
worked computers to have access to an Internet connection. Here are two
ways to share an Internet connection over the network:
164 Part III: Installing a Wireless Network

Connection sharing: All network users access the Internet via one com-
puter that™s specifically set up for doing just that.
A router or an Internet gateway: A router handles the traffic manage-
ment to enable all network users access to the Internet. An Internet gate-
way is a broadband modem with a bundled-in router. A wireless Internet
gateway adds an access point (AP) to the mix.



Connection sharing
Windows 98 and later versions of Windows enable Internet connection shar-
ing, as does Mac OS X. When using this method to share an Internet connec-
tion, each computer in a wired or wireless network is set up to connect to the
Internet through the computer that™s connected to the modem that™s con-
nected to the Internet. The disadvantage with this system is that you can™t
turn off or remove the computer that™s connected to the modem without also
disconnecting all computers from the Internet. In addition, simultaneous
usage (several people on the network using the Internet at once) can slow
down the computer providing the connection.

Mac OS 9 and Mac OS X v. 10.2 (called Jaguar) or later include a program for
the Apple AirPort system called AirPort Software Base Station. The Base
Station enables you to share an Internet connection by creating a software-
based wireless Base Station in one of the computers on your network. Other
computers on the network with wireless network adapters can access the
Internet through the soft Base Station. Again, the computer that™s running
this Base Station software has to be turned on for the other computers in the
wireless network to gain access to the Internet, and this Base Station com-
puter is affected by the same performance degradation as in the preceding
scenario.



Routers and gateways
By connecting a router between the broadband modem and your home net-
work, all computers on the network can access the Internet without going
through another computer. The Internet connection no longer depends on
any computer on the network.

The types of routers used in homes are often cable/DSL routers. These
devices are also DHCP servers and also include Network Address Translation
(NAT) services. The most popular type of device for sharing an Internet
connection over a home network, often described as a wireless gateway,
combines the features of a router, a DHCP server, a NAT server, and the
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Chapter 9: Setting Up Internet Sharing

capabilities of a wireless AP. In addition to wireless connectivity, most of
these devices also have several Ethernet ports for connecting computers
with network cable, giving you the flexibility of adding wired devices and
expanding your network connections. Each computer connects to the
wireless gateway; the wireless gateway device connects to the broadband
(usually DSL or cable) modem; and the modem connects to the Internet.

The nature of the Internet and Transmission Control Protocol/Internet
Protocol (TCP/IP) networking requires that every machine or device connect-
ing has to have a unique IP address. For information to get to its proper desti-
nation, every piece of information has to contain the IP address that it came
from and the IP address that it™s going to for it to get from one point to
another.

A NAT server allows for the conversion of one IP address to one or many
other IP addresses. This means that a whole group of computers can look like
just one computer to the rest of the Internet. This is becoming more the norm
in both home and corporate networks these days because we have many
more computers and devices using the Internet today than we have IP
addresses to give them. Connecting to an Internet service provider (ISP) will
typically deliver one IP address to the device performing the connection.
This is true for dialup, cable, and satellite modems as well as DSL. That IP
address is used by the computer or Internet gateway that the modem con-
nects to.

If you have one computer, getting an IP address assigned to your computer is
very simple because the modem device delivers the IP address to the com-
puter, and the computer uses that address as its own and connects to the
Internet. If you have more than one computer or device to connect, you have
to share that one IP address that the modem receives among those machines.
NAT creates an internal addressing scheme using one of the reserved IP
address ranges that the Internet does not use. (192.168.x.x is one of two Class
B networks that are used internally to home or office networks using NAT.)
Many companies and almost all cable/DSL routers use these address ranges
on the networks behind them. In many cases, it™s a given that the IP address
of the cable/DSL router will be 192.168.1.1 or 192.168.2.1, depending on which
address NAT is configured to use.

After the address translation is in place, a DHCP server then assigns the local
IP addresses for all the devices connected inside your home network. The
Internet gateway™s NAT function enables all computers connected to the
Internet through the Internet gateway device to share the same IP address on
the Internet. Figure 9-1 depicts a wireless home network that uses an Internet
gateway providing NAT and DHCP to share Internet access to three comput-
ers over wireless connections and to two more over wired connections.
166 Part III: Installing a Wireless Network



Wired PC
198.162.1.6 Internet
Wireless
PC
Figure 9-1:
198.162.1.5
A wireless
home
network
Wireless
using a 209.211.202.11 is delivered
PC
wireless from the modem.
198.162.1.4
Internet
Cable/DSL
gateway modem
device
shares an
Internet Wireless
Wireless
connection Internet
PC
with wired gateway/DHCP
198.162.1.3
(198.162.1.1)
and
Wired PC
wireless
198.162.1.2
computers.




Sharing dialup Internet connections
You can use connection sharing and a home network to share a single dialup
connection. This would be especially practical if you have a dedicated tele-
phone line for Internet access. You can use a dialup modem to connect to the
Internet on the dedicated line, leave the connection running, and then share
this connection with all the computers on your home network so that they
can access the Internet.

Similarly, if you purchase an Internet gateway that includes a dialup modem,
you can use the gateway to share a dialup connection. You can connect the
gateway to the Internet using the dialup modem and then use the gateway™s
router feature to share this connection with all the networked computers.
Some Internet gateways (usually those designed for small businesses) com-
bine both a broadband (DSL usually) modem and a dialup modem in one box.
You can use the dialup modem as a backup system if your broadband con-
nection ever goes down.

Apple™s AirPort Base Station does include a dialup modem (standard on older
AirPort Base Stations and optional on the AirPort Extreme Base Station) and
also includes ISP logon features that can successfully connect to AOL, but
you still need multiple AOL accounts for multiple users to access the Internet
simultaneously through AOL.
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Chapter 9: Setting Up Internet Sharing


Obtaining an IP Address Automatically
For the computers in your home network to communicate effectively with
one another, whether connected to the network by wire or wirelessly, all
computers must have IP (network) addresses on the same subnet ” the net-
work address equivalent of having house addresses in the same Zip code or
having phone numbers in the same area code. For example, the local IP
addresses 192.168.0.1 and 192.168.0.55 are on the same subnet, but the IP
addresses 192.168.0.1 and 192.168.1.55 are not. Note that the number after
the second dot (referred to by computer geeks as the third octet) must be the
same for the address to be in the same subnet. In addition, all must have the
same subnet mask, which is typically 255.255.255.0.

A subnet (or subnetwork) is simply a portion of a network (like your home
wireless network) that has been portioned off and grouped together as a
single unit. When you use a wireless Internet gateway, all your computers are
placed into the same subnet. The single IP address assigned to your modem
can provide Internet access to all the computers on the subnet. The actual
numbers used to identify the subnet are the subnet mask. As we mention pre-
viously, you™ll typically use 255.255.255.0 as your subnet mask. The important
thing is to ensure that all the computers and devices connected to your wire-
less home network also have that same subnet mask assigned to them ” oth-
erwise, they won™t connect to the Internet. Most of the time, you don™t have
to do anything here because your computer should have this subnet mask
set up by default.

You can manually assign the IP address of each device connected to the net-
work, but (luckily) you usually don™t have to worry about using this feature.
For some applications, such as gaming or videoconferencing ” and for some
non-computer devices on your network such as game consoles ” you might
have to enter a static IP address into your router™s configuration. In the
majority of cases, however ” that is, for most normal PC Internet connec-
tions ” the DHCP server built into cable/DSL routers and wireless Internet
gateways takes care of IP address assignments for you.

One of the most common errors when setting up a home network comes from
using a router providing DHCP and NAT and combining it with a wireless
access point that also provides DHCP and NAT. If you™re using two devices
rather than a combined one, you need to be sure that you only enable the
DHCP and NAT services on the router that™s connected to the modem device.
The AP should have an option when it™s configured to be set up as a bridge
device. This turns off its services and allows the device to be a wireless con-
duit to the network created by the DHCP and NAT services of the Internet
gateway. Failing to do this can result in a segmented home network in which
the wired devices cannot share with the wireless devices, or the wireless
devices won™t be able to share with anything but themselves and will be
unable to access the Internet through your ISP.
168 Part III: Installing a Wireless Network

Suppose that you install a network adapter (refer to Chapter 7), launch your
Web browser, and try to reach the Internet. If you then have problems
(assuming that everything else is connected and other computers on the net-
work can successfully access the Internet), perhaps the IP address wasn™t
properly assigned to the adapter. Before you start panicking, try shutting
down and restarting the computer. Often, restarting the computer will cause
the network adapter to properly obtain an IP address from the network™s
DHCP server. If you still can™t reach the Internet, follow the instructions in
this section to configure the network adapter to automatically obtain an IP
address. (Also check out Chapter 18, where we cover some basic trou-
bleshooting techniques for home wireless networks.)

Configuring a device on the network (wired or wireless) to automatically
obtain an IP address from a DHCP server is very easy. Throughout the rest of
this section, we outline the steps necessary for automatically obtaining an IP
address from a DHCP server for various operating systems: Windows
9x/2000/XP and Mac OS X. After you complete the applicable procedure, the
DHCP server leases a local IP address to the device that you are configuring,
enabling it to communicate with other IP devices on the network.

Sometimes you have to restart your computer to successfully achieve the
desired result.

When a DHCP server leases an IP address, the server will not assign that IP
address to another device until the lease runs out or the device that is leas-

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