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Figure 4-8 to review the bit separation between network and host information in classful addressing.Figure 11-1 offers some example IP addresses separated into network and host informati

too, increasingly rely on the Internet for purchasing and data-gathering operations.

In previous chapters, you learned that the Internet depends on the TCP/IP suite of protocols,

as do a number of network operating systems Because of the increasing popularity of the net, having TCP/IP expertise can pave the way to a lucrative, challenging, and rewarding career

Inter-In Chapter 4, you learned about core protocols and subprotocols in the TCP/IP protocol suite,addressing schemes, and host and domain naming You also learned that TCP/IP is a complexand highly customizable protocol suite This chapter builds on these basic concepts, examin-ing how TCP/IP-based networks are designed and analyzed It also describes the services andapplications that TCP/IP-based networks commonly support If you are unclear about the con-cepts related to IP addressing or binary-to-decimal conversion, take time to review Chapter 4before reading this chapter

Designing TCP/IP-Based Networks

By now, you understand that most modern networks rely on the TCP/IP protocol suite, notonly for Internet connectivity, but also for transmitting data over private connections Beforeproceeding with TCP/IP network design considerations, it’s useful to briefly review someTCP/IP fundamentals For example, you have learned that IP is a routable protocol, and that

on a network using TCP/IP each interface is associated with a unique IP address Some nodesmay use multiple IP addresses For example, on a router that contains two NICs, each NICcan be assigned a separate IP address Or, on a Web server that hosts multiple Web sites—such

as one operated by an ISP—each Web service associated with a site can have a different IPaddress

IP addresses consist of four 8-bit octets (or bytes) that can be expressed in either binary (forexample, 10000011 01000001 00001010 00100100) or dotted decimal (for example,131.65.10.36) notation Many networks assign IP addresses and host names dynamically,using DHCP, rather than statically You also know that every IP address can be associated with

a network class—A, B, C, D, or E (though Class D and E addresses are reserved for specialpurposes) A node’s network class provides information about the segment or network to whichthe node belongs The following section explains how network and host information in an IPaddress can be manipulated to subdivide networks into smaller segments

Subnetting separates a network into multiple logically defined segments, or subnets Networksare commonly subnetted according to geographic locations (for example, the floors of a build-ing connected by a LAN, or the buildings connected by a WAN), departmental boundaries, ortechnology types (for example, Ethernet or Token Ring) Where subnetting is implemented,each subnet’s traffic is separated from every other subnet’s traffic A network administratormight separate traffic to:

◆ Enhance security—Subnetworks must be connected via routers or other Layer 3

devices As you know, these devices do not retransmit incoming frames to all othernodes on the same segment (as a hub does) Instead, they forward frames only asnecessary to reach their destination Because every frame is not indiscriminatelyretransmitted, the possibility for one node to tap into another node’s transmissions isreduced

◆ Improve performance—For the same reason that subnetting enhances security, it also

improves performance on a network When data is selectively retransmitted, essary transmissions are kept to a minimum In the case of Ethernet networks, sub-netting is useful for limiting the amount of broadcast traffic—and therefore theamount of potential collisions—by decreasing the size of each broadcast domain

unnec-The more efficient use of bandwidth results in better overall network performance

◆ Simplify troubleshooting—For example, a network administrator might subdivide an

organization’s network according to geography, assigning a separate subnet to thenodes in the downtown office, west-side office, and east-side office of her company.Suppose one day the network has trouble transmitting data only to a certain group

of IP addresses—those located on the west-side office subnet When ing, rather than examining the whole network for errors or bottlenecks, the networkadministrator needs only to see that the faulty transmissions are all associated withaddresses on the west-side subnet to know that she should zero in on that subnet

troubleshoot-To understand how subnetting is implemented, it’s necessary to first review IP addressing ventions on a network that does not use subnetting

con-Classful Addressing

In Chapter 4, you learned about the first and simplest type of IP addressing, which is known as

classful addressing because it adheres to network class distinctions In classful addressing, only Class

A, Class B, and Class C addresses are recognized Recall that all IP addresses consist of networkand host information In classful addressing, the network information portion of an IP address (thenetwork ID) is limited to the first 8 bits in a Class A address, the first 16 bits in a Class B address,and the first 24 bits in a Class C address Host information is contained in the last 24 bits for a Class

A address, the last 16 bits in a Class B address, and the last 8 bits in a Class C address Refer to

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Figure 4-8 to review the bit separation between network and host information in classful addressing.Figure 11-1 offers some example IP addresses separated into network and host information accord-ing to the classful addressing convention.

FIGURE 11-1 Example IP addresses with classful addressing

Adhering to a fixed network ID size ultimately limits the number of hosts a network can include.For example, leasing an entire Class C network of addresses gives you only 254 usable IP addresses

In addition, using classful addressing makes it difficult to separate traffic from various parts of a work As you have learned, separating traffic offers many practical benefits For example, if an orga-nization used an entire Class B network of addresses, it could have up to 65,534 hosts all on onenetwork segment Imagine the challenges involved in managing such a highly populated net-work, not to mention the poor performance that would result In 1985, because of the difficulty

net-of managing a whole network class net-of addresses and the dwindling supply net-of usable IP addresses,computer scientists introduced subnetting

Depending on the source, you may find the term network ID used interchangeably

with the terms network number or network prefix.

Each network class is associated with a default subnet mask, as shown in Table 11-1 For ple, by default, a Class A address’s first octet (or 8 bits) represents network information and is

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composed of all 1s (Recall that an octet composed of all 1s in binary notation equals 255 indecimal notation An octet composed of all 0s in binary notation equals 0 in decimal nota-tion.) That means that if you work on a network whose hosts are configured with a subnet mask

of 255.0.0.0, you know that the network is using Class A addresses and, furthermore, that it isnot using subnetting, because 255.0.0.0 is the default subnet mask for a Class A network

Table 11-1 Default subnet masks

Number of Bits Default Subnet Network Default Subnet Mask Used for Network Mask (Dotted

A 11111111 00000000 00000000 00000000 8 255.0.0.0

B 11111111 11111111 00000000 00000000 16 255.255.0.0

C 11111111 11111111 11111111 00000000 24 255.255.255.0

To calculate a host’s network ID given its IP address and subnet mask, you follow a logical

process of combining bits known as ANDing In ANDing, a bit with a value of 1 plus another

bit with a value of 1 results in a 1 A bit with a value of 0 plus any other bit results in a 0 Ifyou think of 1 as “true” and 0 as “false,” the logic of ANDing makes sense Adding a true state-ment to a true statement still results in a true statement But adding a true statement to a falsestatement results in a false statement ANDing logic is demonstrated in Table 11-2, which pro-vides every possible combination of having a 1 or 0 bit in an IP address or subnet mask

Table 11-2 ANDing

An example host IP address, its default subnet mask, and network ID are shown in Figure

11-2 in both binary and dotted decimal notation Notice that the IP address’s fourth octet couldhave been composed of any combination of 1s and 0s, and the network ID’s fourth octet wouldstill be all 0s

FIGURE 11-2 Example of calculating a host’s network ID

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At this point, you should understand how to determine a host’s network ID given its IP addressand subnet mask This section explained how to apply ANDing logic to an IP address plus a

default subnet mask, but it works just the same way for networks that are subnetted and have

different subnet masks, as you will soon learn Before learning how to create subnets, however,

it is necessary to understand the types of addresses that cannot be used as subnet masks or hostaddresses

Reserved Addresses

Certain types of IP addresses cannot be assigned to a network interface on a node or used assubnet masks Instead, these IP addresses are reserved for special functions One type ofreserved address should be familiar to you already—that is, the network ID In a network ID,

as you know, bits available for host information are set to 0 Therefore, a workstation on theexample network used in Figure 11-2 could not be assigned the IP address 199.34.89.0, becausethat address is the network ID When using classful addressing, a network ID always endswith an octet of 0 (and may have additional, preceding octets equal to 0) However, when sub-netting is applied and a default subnet mask is no longer used, a network ID may have otherdecimal values in its last octet(s)

Another reserved IP address is the broadcast address for a network or segment In a broadcastaddress, the octet(s) that represent host information are set to equal all 1s, or in decimal nota-tion, 255 In the example in Figure 11-2, the broadcast address would be 199.34.89.255 If aworkstation on that network sent a message to the address 199.34.89.255, it would be issued

to every node on the segment

Because the octets equal to 0 and 255 are reserved, only the numbers 1 through 254 can beused for host information in an IP address Thus, on a network that followed the example inFigure 11-2, the usable host addresses would range from 199.34.89.1 to 199.34.89.254 If yousubnetted this network, the range of usable host addresses would be different The next sec-tion describes how subnets are created and how you can determine the range of usable hostaddresses on a subnet

Subnetting Techniques

Subnetting breaks the rules of classful addressing To create subnets, some of an IP address’sbits that in classful addressing would represent host information are changed to represent net-work information instead By making bits that previously were used for host information rep-resent network information, you reduce the number of bits available for identifying hosts.Consequently, you reduce the number of usable host addresses per subnet The number of hostsand subnets available after subnetting is related to how many host information bits you use (orborrow, as network professionals like to say) for network information Table 11-3 illustratesthe numbers of subnets and hosts that can be created by subnetting a Class B network Noticethe range of subnet masks that can be used instead of the default Class B subnet mask of255.255.0.0 Also compare the listed numbers of hosts per subnet to the 65,534 hosts available

on a Class B network that does not use subnetting

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Table 11-3 Class B subnet masks

to a lower number of hosts per subnet than the number available to Class B subnets

Table 11-4 Class C subnet masks

where n= the number of bits in the subnet mask that must be switched from 0 to 1

andY= the number of subnets that result

Notice that this formula subtracts 2 from the total number of possible subnets—that is, from the culation of 2 to the power of the number of the bits that equal 1 That’s because in traditional sub-netting, bit combinations of all 0s or all 1s are not allowed for identifying subnets; just as host addressesending in all 0s or all 1s are not allowed because of addresses reserved for the network ID and broad-cast transmissions (However, in the next section of this chapter you learn why this equation doesn’tapply to all modern networks.)

cal-Because you want six separate subnets, the equation becomes 6=2n-2 Because 6+2 equals 8 and8=23, you know that the value of n equals 3 Thus, you need to change three additional subnetmask bits from 0 to 1 That means that rather than using the default subnet mask, in whichthe first 24 bits indicate the position of network information, you would use a subnet mask of

11111111 111111111 11111111 11100000, in which the first 27 bits indicate the position ofnetwork information Converting from binary to the more familiar dotted decimal notation,this subnet mask becomes 255.255.255.224 When you configure the TCP/IP properties ofclients on your network, you would specify this subnet mask

Now that you have calculated the subnet mask, you still need to assign IP addresses to nodesbased on your new subnetting scheme Recall that you have borrowed three bits from what used

to be host information in the IP address That leaves five bits available in the last octet of yourClass C addresses to identify hosts Adding the values of the last five bits, 16 + 8 + 4 + 2 + 1,equals 31, for a total of 32 potential addresses (0 through 31) However, as you have learned,one address is reserved for the network ID and cannot be used Another address is reserved forthe broadcast ID and cannot be used Thus, using five bits for host information allows a max-imum of 30 different host addresses for each of the six subnets So, in this example, you canhave a maximum of 6 x 30, or 180, unique host addresses on the network

Table 11-5 lists the network ID, broadcast address, and usable host addresses for each of the sixsubnets in this example Class C network Together, the additional bits used for subnet infor-

mation plus the existing network ID are known as the extended network prefix The extended

network prefix for each subnet is based on which of the additional (borrowed) network mation bits are set to equal 1 For example, in subnet number 1, only the third bit of the three

infor-is set to 1, making the last octet of the extended network prefix 00100000, or in decimal tion, 32 In subnet number 2, only the second bit is set to 1, making the last octet of the extendednetwork prefix 01000000, or 64 In Table 11-5, the three bits borrowed from the host informa-tion portion of the Class C address (to indicate network information) are underlined

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Class A, Class B, and Class C networks can all be subnetted But because each class reserves

a different number of bits for network information, each class has a different number of hostinformation bits that can be used for subnet information The number of hosts and subnets onyour network will vary depending on your network class and the way you use subnetting Enu-merating the dozens of subnet possibilities based on different arrangements and network classes

is beyond the scope of this book However, several Web sites provide excellent tools that help

you calculate subnet information One such site is www.subnetmask.info.

If you use subnetting on your LAN, only your LAN’s devices need to interpret your devices’subnetting information Routers external to your LAN, such as those on the Internet, pay atten-tion to only the network portion of your devices’ IP addresses when transmitting data to them

As a result, devices external to a subnetted LAN (such as routers on the Internet) can directdata to those LAN devices without interpreting the LAN’s subnetting information

Table 11-5 Subnet information for six subnets in an example Class C network

Figure 11-3 illustrates a situation in which a LAN has been granted the Class C range ofaddresses that begin with 199.34.89 The network administrator has subnetted this Class Cnetwork into six smaller networks with the network IDs listed in Table 11-5 As you know,routers connect different network segments via their physical interfaces In the case of subnet-ting, a router must interpret IP addresses from different subnets and direct data from one sub-net to another Each subnet corresponds to a different port on the router.

FIGURE 11-3 A router connecting several subnets

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When a router on the internal LAN needs to direct data from a machine with the IP address of199.34.89.73 to a machine with the IP address of 199.34.89.114, its interpretation of the worksta-tions’ subnet masks (255.255.255.224) plus the host information in the IP addresses tells the routerthat they are on different subnets The router forwards data between the two subnets (or ports) Inthis figure, the devices connecting subnets to the router are labeled switches, but they could also berouters, bridges, or hubs Alternatively, nodes having different extended network prefixes could bedirectly connected to the router so that each subnet is associated with only one device, though this is

an unlikely configuration

When a server on the Internet attempts to deliver a Web page to the machine with IP address199.34.89.73, however, the Internet router does not use the subnet mask information It only knowsthat the machine is on a Class C network beginning with a network ID of 199.34.89 That’s all theinformation it needs to reach the organization’s router After the data enters the organization’s LAN,the LAN’s router then interprets the subnet mask information as if it were transmitting data inter-nally to deliver data to the machine with IP address 199.34.89.73 Because subnetting does not affecthow a device is addressed by external networks, a network administrator does not need to informInternet authorities about new segments created via subnetting

You have learned how to subdivide a network into multiple smaller segments through subnetting.Next, you’ll learn about more contemporary variations on this method

CIDR (Classless Inter-Domain Routing)

By 1993, the Internet was growing exponentially, and the demand for IP addresses was growing with

it The IETF (Internet Engineering Task Force) recognized that additional measures were sary to increase the availability and flexibility of IP addresses In response to this need, the IETF

neces-devised CIDR (Classless Inter-Domain Routing, pronounced cider), which is sometimes called

classless routing or supernetting CIDR is not exclusive of subnetting; it merely provides additional

ways of arranging network and host information in an IP address In CIDR, conventional networkclass distinctions do not exist

For example, the previous section described subdividing a Class C network into six subnets of 30addressable hosts each To achieve this, the subnet boundary (or length of the extended network pre-fix) was moved to the right—from the default 24thbit to the 27thbit—into what used to be the hostinformation octet In CIDR, a subnet boundary can move to the left Moving the subnet boundary

to the left allows you to use more bits for host information and, therefore, generate more usable IPaddresses on your network A subnet created by moving the subnet boundary to the left is known as

a supernet Figure 11-4 contrasts examples of a Class C supernet mask with a subnet mask.

Notice that in Figure 11-4, 27 bits are used for network information in the subnet mask, whereasonly 22 bits are used for network information in the supernet mask

Suppose that you have leased the Class C range of addresses that shares the network ID199.34.89.0 and, because of growth in your company, you need to greatly increase the number

of host addresses this network allows by default By changing the default subnet mask of255.255.255.0 (11111111 11111111 11111111 00000000) to 255.255.252.0 (11111111

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11111111 11111100 00000000), as shown in Figure 11-4, you can make available two extrabits for host information Adding the values of the last 10 bits, 512 + 256 + 128 + 64 + 32 +

16 + 8 + 4 + 2 + 1, equals 1023, which leads to 1024 (0 through 1023) potential host addresses

on each subnet However, as you know, two addresses are reserved and therefore unusable ashost addresses Thus, the actual number of host addresses available on this subnet is 1022

In this example, you have subtracted information from the host portion of the IP address.Therefore, the IP addresses that result from this subnetting scheme will be different from the

IP addresses you would use if you had left the network ID untouched (as in the subnettingexample used in the previous section) The calculation for the new network ID is shown in Fig-ure 11-5 For this example subnetted Class C network, the potential host addresses fall in therange of 199.34.88.1 to 199.34.91.254 The broadcast address is 199.34.91.255

FIGURE 11-4 Subnet mask and supernet mask

FIGURE 11-5 Calculating a host’s network ID on a supernetted network

With CIDR also came a new shorthand for denoting the position of subnet boundaries, known as

CIDR notation (or slash notation) CIDR notation takes the form of the network ID followed by

a forward slash (/), followed by the number of bits that are used for the extended network prefix Forexample, for the Class C network whose network ID is 199.34.89.0 and which was divided into sixsubnets, the slash notation would be 199.34.89.0/27, because 27 bits of the subnets’ addresses are usedfor the extended network prefix The CIDR notation for the Class C network used as an example of

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supernetting earlier in this section would be 199.34.89.0/22 In CIDR terminology, the forward slash,plus the number of bits used for the extended network prefix—for example, “/22”—is known as a

CIDR block.

To take advantage of classless routing, your network’s routers must be able to interpret IP addressesthat don’t adhere to conventional network class parameters Routers that rely on older routing pro-tocols, such as RIP, are not capable of interpreting classless IP addresses

Internet Gateways

Gateways are a combination of software and hardware that enable two different network ments to exchange data A gateway facilitates communication between different networks orsubnets Because one device on the network cannot send data directly to a device on anothersubnet, a gateway must intercede and hand off the information Every device on a TCP/IP-

seg-based network has a default gateway—that is, the gateway that first interprets its outbound

requests to other subnets, and then interprets its inbound requests from other subnets

A gateway is analogous to your local post office Your post office gathers your outbound mail anddecides where to forward it It also handles your inbound mail on its way to your mailbox Just as alarge city has several local post offices, a large organization will have several gateways to route trafficfor different groups of devices Each node on the network can have only one default gateway; thatgateway is assigned either manually or automatically (in the latter case, through a service such asDHCP) Of course, if your network includes only one segment and you do not connect to the Inter-net, your devices would not need a default gateway because traffic would not need to cross the net-work’s boundary

In many cases, a default gateway is not a separate device, but rather a network interface on a

router For this reason, you may hear the term default router used to refer to a default

way By using a router’s network interfaces as gateways, one router can supply multiple ways Each default gateway is assigned its own IP address In Figure 11-6, workstation10.3.105.23 (workstation A) uses the 10.3.105.1 gateway to process its requests, and worksta-tion 10.3.102.75 (workstation B) uses the 10.3.102.1 gateway for the same purpose

gate-An Internet gateway is usually assigned an IP address that ends with an octet of 1

NOTE

Default gateways may connect multiple internal networks, or they may connect an internal work with external networks, such as WANs or the Internet As you have learned, routers thatconnect multiple networks must maintain a routing table to determine where to forward infor-mation When a router is used as a gateway, it must maintain routing tables as well

net-The Internet contains a vast number of routers and gateways If each gateway had to trackaddressing information for every other gateway on the Internet, it would be overtaxed Instead,

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each handles only a relatively small amount of addressing information, which it uses to ward data to another gateway that knows more about the data’s destination Like routers on aninternal network, Internet gateways maintain default routes to known addresses to expedite

for-data transfer The gateways that make up the Internet backbone are called core gateways.

NAT (Network Address Translation)

Default gateways can also be used to “hide” the IP numbers assigned within an organizationand keep its devices’ IP addresses secret from a public network (such as the Internet) Hiding

IP addresses allows network managers more flexibility in assigning addresses Clients behind

a gateway may use any IP addressing scheme, regardless of whether it is legitimately recognized

by the Internet authorities But once those clients need to connect to the Internet, they musthave a legitimate IP address to exchange data When the client’s transmission reaches thedefault gateway, the gateway assigns the client’s transmission a valid IP address After the trans-mission has been terminated, that IP address becomes available for another gateway transmis-

sion This process is known as NAT (Network Address Translation).

One reason for hiding IP addresses is to add a marginal amount of security to a private work when it is connected to a public network Because a transmission is assigned a new IPaddress each time it reaches the public sphere, those outside an organization cannot trace theorigin of the transmission back to the specific network node that sent it However, the IPaddress assigned to a transmission by the gateway must be an Internet-authorized IP address;thus, it can be traced back to the organization that leased the address

net-FIGURE 11-6 The use of default gateways

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Another reason for using NAT is to enable a network administrator to develop her own work addressing scheme that does not conform to a scheme dictated by ICANN For exam-ple, suppose you are the network administrator for a private elementary school You maintainthe school’s entire network, which, among other things, includes 200 client workstations Sup-pose half of these clients are used by students in the classrooms or library and half are usedexpressly by staff To make your network management easier, you might decide to assign eachstudent workstation an IP address whose first octet begins with the number 10 and whose sec-ond octet is the number of the classroom where the computer is located For example, a stu-dent workstation in room 235 might have an IP address of 10.235.1.12 You might then assigneach staff workstation an IP address whose first octet is the number 50 and whose second octet

net-is the number of the employee’s office or classroom For example, the principal’s workstation,which is located in his office in Room 135, might have an IP address of 50.135.1.10 These IPaddresses would be used strictly for communication between devices on the school’s network.When staff or students wanted to access the Internet, however, you would need to have at leastsome IP addresses that would be legitimate for use on the Internet Now suppose that, becausethe school has limited funds and does not require that all clients be connected to the Internet

at all times, you decide to lease only 20 IP numbers from your ISP You then configure a way to translate your internal addresses to addresses that can be used on the Internet Eachtime a client attempts to reach the Internet, the gateway would replace its source address field

gate-in the data packets with one of the 20 legitimate IP addresses Figure 11-7 depicts how theNAT works

FIGURE 11-7 NAT through an Internet gateway

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You have learned that NAT separates private and public transmissions on a TCP/IP network In thenext section, you will learn about a service of some Windows operating systems that enables one com-puter to provide NAT for other computers on the same network.

ICS (Internet Connection Sharing)

On a small or home network, multiple computers often share a single Internet connection Inprevious chapters, you learned that this can be achieved by using a SOHO router or smallswitch to which each networked computer is connected Microsoft offers another alternativefor sharing Internet access on computers running the Windows 98, Me, 2000, or 32-bit ver-

sion of the XP operating system, ICS (Internet Connection Sharing) Using ICS, a puter with Internet access, called the ICS host, is configured to translate requests to and from

com-the Internet on behalf of ocom-ther computers on com-the network To do this, it acts as a DHCPserver, DNS resolver, and NAT gateway for clients on its LAN The ICS host requires two net-work connections: one that connects to the Internet, which could be dial-up, DSL, ISDN, orbroadband cable, and one that connects to the LAN If the network uses a dial-up connection

to the Internet, the ICS host connects to the Internet on demand—that is, when other puters on the network issue a request to the Internet

com-When ICS is enabled on a LAN, the network adapter on the ICS host that connects to theLAN is assigned an IP address of 192.168.0.1 Clients on the small office or home office LANmust be set up to obtain IP addresses automatically The ICS host then assigns clients IPaddresses in the range of 192.168.0.2 through 192.168.0.255 If you are already using this range

of addresses on your network (for example, in a NAT scheme), you might experience problemsestablishing or using ICS

To enable ICS on an ICS host:

1. Log on to the ICS host computer as Administrator or as a user with equivalent leges

privi-2. Verify that a connection between the ICS host computer and the Internet is tional

opera-3. Click Start, then click My Network Places The My Network Places window opens.

4. Click View Network Connections The Network Connections window opens.

5. Right-click the Local Area Connection icon that represents your ICS host computer’s

connection to the Internet, and then click Properties The Local Area Connection

Properties dialog box opens

6. Click the Advanced tab.

7. Under Internet Connection Sharing, check Allow other network users to connect through this computer’s Internet connection By default, the Allow other network

users to control or disable the shared Internet connection option is also checked, asshown in Figure 11-8 However, this option can be deselected if you do not wantother computers on the network to be able to control the ICS properties on the ICShost

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