IP Addresses 399
Figure 7-27 Broadcast Address
Figure 7-28 Network Address
In a Class B network address, the first two octets, written as dotted-decimal numbers,
are assigned by default. The last two octets contain 0s because those 16 bits are for
host numbers and identify devices that are attached to the network. This is called a
unicast address (uni means one). A unicast address points to just one host on the net-
work. The IP address in the example (176.10.0.0) is reserved for the network address
and is never used as an address for any device that is attached to it. An example of
an IP address for a device on the 176.10.0.0 network is 176.10.16.1. In this example,
176.10 is the network address portion, and 16.1 is the host address portion.
To send data to all the devices on a network, a broadcast address is needed. A broadcast
occurs when a source sends data to all devices on a network, as shown in Figure 7-29.
This Class B address is the broadcast address for this network. When packets are received
with this destination address, the data is processed by every computer. To ensure that
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400 Chapter 7: TCP/IP Protocol Suite and IP Addressing
all the other devices on the network process the broadcast, the sender must use a desti-
nation IP address that they can recognize and process. Broadcast IP addresses end with
binary 1s in the entire host part of the address (the Host field).
Figure 7-29 Broadcast Address
For the network 176.10.0.0, where the last 16 bits make up the Host field (or the host
part of the address), the broadcast that is sent to all devices on that network includes a
destination address of 176.10.255.255 (because 255 is the decimal value of an octet
containing 11111111).
Public and Private Addresses
Internet stability depends directly on the uniqueness of publicly used network addresses.
As shown in Figure 7-30, there is an issue with the networking addressing scheme. Both
networks have a network address of 198.150.11.0. When data transmissions reach the
router, which network would it forward to? A scheme such as this one would greatly
increase the amount of network traffic and would defeat a router’s basic function.

Therefore, some mechanism was needed to ensure that addresses were, in fact, unique.
This responsibility originally rested with the InterNIC (Internet Network Information
Center). This organization is now defunct and has been succeeded by the Internet
Assigned Numbers Authority (IANA). IANA carefully manages the remaining supply
of IP addresses to ensure that duplication of publicly used addresses does not occur.
Such duplication would cause instability in the Internet and compromise its capability
to deliver datagrams to networks using the duplicated addresses.
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IP Addresses 401
Figure 7-30 Required Unique Addresses
Public IP addresses are unique. No two machines that connect to a public network can
have the same IP address, because public IP addresses are global and standardized. All
machines connected to the Internet agree to adhere to the system. Public IP addresses
must be obtained from an Internet service provider (ISP) or a registry at some expense.
With the rapid growth of the Internet, public IP addresses were beginning to run out, so
new addressing schemes such as classless interdomain routing (CIDR) and IPv6 were
developed to help solve the problem. CIDR and IPv6 are discussed later.
Another solution that was developed is the use of private IP addresses, as shown in
Table 7-8. As stated previously, Internet hosts require a globally unique IP address.
However, private networks that are not connected to the Internet can use any valid
address, as long as it is unique within the private network. Many private networks
exist alongside public networks. Grabbing “just any address” is strongly discouraged
because that network might eventually be connected to the Internet.
RFC 1918 sets aside three blocks of IP addresses (a single Class A address, a range of
Class B addresses, and a range of Class C addresses) for private, internal use. Addresses
in this range are not routed on the Internet backbone; Internet routers immediately
discard private addresses.
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402 Chapter 7: TCP/IP Protocol Suite and IP Addressing
If you are addressing a nonpublic intranet, a test lab, or a home network, these private

addresses can be used instead of globally unique addresses. Private IP addresses can be
intermixed with public IP addresses, as shown in Figure 7-31, to conserve the number
of addresses used for internal connections.
Figure 7-31 Using Private IP Addresses Within the WAN
Connecting a network to the Internet using private addresses requires translating the
private addresses to public addresses. This translation process is called Network Address
Translation (NAT). A router usually is the device that performs NAT.
Introduction to Subnetting
Another way to conserve IP addresses, like CIDR, IPv6, and private addresses, is the
use of subnetting. This method of dividing full network address classes into smaller
pieces has helped prevent complete IP address exhaustion. Figure 7-32 shows a Class B
network (131.108.0.0) divided into three subnetworks. It is impossible to cover TCP/IP
without mentioning subnetting. As a system administrator, you must understand sub-
netting as a means of dividing and identifying separate networks throughout the LAN.
It is not always necessary to subnet a small network, but for large or extremely large
networks, subnetting is required. Simply stated, subnetting a network means using the
subnet mask to divide the network and break a large network into smaller, more effi-
cient, more manageable segments, or subnets, as shown in Figure 7-33. This is like the
Table 7-8 Private IP Addresses
IP Address Class RFC 1918 Internal Address Range
Class A 10.0.0.0 to 10.255.255.255
Class B 172.16.0.0 to 172.31.255.255
Class C 192.168.0.0 to 192.168.255.255
Site A
Site B
Site C
Site D
207.21.24.0/27
207.21.24.32/27
207.21.24.64/27

207.21.24.96/27
10.0.0.4/30
10.0.0.8/30 10.0.0.12/30
Internet
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IP Addresses 403
American telephone system, which breaks the system into area codes, and then exchange
codes, and finally local numbers. These elements of the phone system are comparable
to network numbers, subnets, and individual host addresses, respectively, in an IP
internetwork.
Figure 7-32 Addressing with Subnets
Figure 7-33 Subnet Addresses
The system administrator must resolve these issues when adding and expanding the
network. It is important to know how many subnet/networks are needed and how
many hosts are allowed to be on each network. With subnetting, the network is not
limited to the standard Class A, B, or C network masks and there is more flexibility in
the network design.
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404 Chapter 7: TCP/IP Protocol Suite and IP Addressing
Subnet addresses include the Class A, Class B, or Class C network portion, plus a Subnet
field and a Host field. These fields are created from the original host portion for the
entire network. The ability to decide how to divide the original host portion into the new
Subnet and Host fields provides addressing flexibility for the network administrator.
To create a subnet address, a network administrator borrows bits from the Host field
and designates them as the Subnet field, as shown in Table 7-9. The minimum number
of bits that can be borrowed is 2. If you were to borrow only 1 bit, to create a subnet,
you would have only a network number (the .0 network) and a broadcast number (the
.255 network). The maximum number of bits that can be borrowed can be any number
that leaves at least 2 bits for the host number. In Table 7-9’s example of a Class C IP
address, bits from the Host field have been borrowed for the Subnet field.

IPv4 Versus IPv6
When TCP/IPa was adopted in the 1980s, it relied on a two-level addressing scheme,
which at the time offered adequate scalability. Unfortunately, the architects of TCP/IP
could not have predicted that their protocol would eventually sustain a global network
of information, commerce, and entertainment. More than 20 years ago, IPv4 offered
an addressing strategy that, although scalable for a time, resulted in an inefficient allo-
cation of addresses.
Table 7-9 Subnet Addresses
Decimal Notation
for First Host
Octet
Number of
Subnets
Number of
Class A Hosts
Per Subnet
Number of
Class B Hosts
Per Subnet
Number of
Class C Hosts
Per Subnet
.192 2 4,194,302 16,382 62
.224 6 2,097,150 8,190 30
.240 14 1,048,574 4,094 14
.248 30 524,286 2,046 6
.252 62 262,142 1,022 2
.254 126 131,070 510 —
.255 254 65,534 254 —
Lab Activity IP Addressing Basics

This exercise helps you develop an understanding of IP addresses and how
TCP/IP networks operate.
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IP Addresses 405
Class A and B addresses make up 75 percent of the IPv4 address space, as shown in
Figure 7-34, but a relative handful of organizations (fewer than 17,000) can be assigned
a Class A or B network number. Class C network addresses are far more numerous than
Class A and Class B addresses, but they account for only 12.5 percent of the possible
4 billion IP addresses.
Figure 7-34 IPv4 Address Allocation
Unfortunately, Class C addresses are limited to 254 hosts, not meeting the needs of
larger organizations that cannot acquire a Class A or B address. Even if there were
more Class A, B, and C addresses, too many network addresses would cause Internet
routers to grind to a halt under the weight of the enormous routing tables required to
store the routes to reach the networks.
As early as 1992, the IETF identified two specific concerns:
■ Exhaustion of the remaining, unassigned IPv4 network addresses—At the time,
the Class B space was on the verge of depletion.
■ The rapid and substantial increase in the size of Internet routing tables because
of the Internet’s growth—As more Class C networks came online, the resulting
flood of new network information threatened the capability of Internet routers to
cope effectively.
Over the past two decades, numerous extensions to IPv4 have been developed that are
specifically designed to improve the efficiency with which the 32-bit address space can
be used. Two of the more important are subnet masks and CIDR.
Meanwhile, an even more extensible and scalable version of IP, IPv6, has been defined
and developed. IPv6 uses 128 bits rather than the 32 bits currently used in IPv4, as
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406 Chapter 7: TCP/IP Protocol Suite and IP Addressing
shown in Figure 7-35. IPv6 uses hexadecimal numbers to represent the 128 bits. It pro-

vides 16 billion IP addresses (3.4 × 10
38
addresses). This version of IP should provide
sufficient addresses for future communication needs.
Figure 7-35 IPv4 and IPv6
The IPv6 shorthand representation of the 128 bits uses eight 16-bit numbers, shown as
four hexadecimal digits, as shown in Figure 7-36. The groups of four hex digits are
separated by colons. If there are leading 0s in the hex digits, they may be omitted.
Figure 7-36 IPv4 and IPv6
After years of planning and development, IPv6 is slowly being implemented in select
networks. Eventually, IPv6 might replace IPv4 as the dominant Internet protocol.
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IP Address Assignment, Acquisition, and Hierarchy 407
IP Address Assignment, Acquisition, and Hierarchy
This section discusses how network devices obtain IP addresses. For a network to keep
functioning, the IP addresses must be assigned according to a specific hierarchy. How
and why this is done are discussed in the following section. IP addresses can be assigned
either statically or dynamically. Both methods are covered here.
Obtaining an Internet Address
For a host on a network to function on the Internet, it needs to obtain a globally unique
address. A host’s physical or MAC address is only locally significant. Being locally sig-
nificant means that the address can only identify the host in its own LAN. It has no
meaning to any device that is not in that LAN.
IP is the most widely used global addressing scheme. It is a hierarchical addressing
scheme that allows individual addresses to be associated and treated as groups, as
shown in Figure 7-37. These groups of addresses allow efficient transfer of data across
the Internet.
Figure 7-37 Internet Address Hierarchy
There are essentially two methods for assigning IP addresses—static addressing and
dynamic addressing. The next few sections cover static and dynamic addressing. Regard-

less of which addressing scheme is chosen, no two interfaces can have the same IP
address. This would cause a conflict that might cause both the hosts involved not to
operate properly.
1.0.0.0 2.0.0.0 3.0.0.0 10.0.0.0 11.0.0.0 255.0.0.0

10.1.0.0 10.2.0.0 10.3.0.0
10.255.0.0

10.2.1.0 10.2.2.0 10.2.3.0 10.2.255.0

10.255.1.0 10.255.2.0 10.255.3.0 10.255.255.0

10.2.2.1 10.2.2.2 10.2.2.3

10.2.255.0 10.255.2.1 10.255.2.2 10.255.2.3 10.255.2.255

Internet Address
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408 Chapter 7: TCP/IP Protocol Suite and IP Addressing
Static Assignment of an IP Address
When IP addresses are assigned statically, each device must be configured with an IP
address. Each operating system has its own way of configuring TCP/IP. This method
requires records of the address assignments to be kept, because problems can occur
in a network if duplicate IP addresses are used. Some operating systems, such as Win-
dows 95 and Windows NT, send an ARP request to check for a duplicate IP address
when they attempt to initialize TCP/IP. If a duplicate is discovered, the operating system
does not initialize TCP/IP and generates an error message. Not all operating systems
identify duplicate IP addresses. This again emphasizes the need for good record-keeping.
The main reason that a device would be assigned a static IP address is if the device needs
to be referenced by other devices. A good example is a web server. If a web server got a

new IP address each time it started up, it would be difficult to find the web server. As
an example of this address changing, if a city were to constantly change street names
and building addresses, maps would no longer help you locate a particular building. If
an address changes, it is no longer easy to return to the location. If a building is diffi-
cult to get to, people will stop trying to locate it.
Certain types of devices need to maintain a static IP address. Web servers, network
printers, application servers, and routers are good examples of devices that require
permanent IP addresses.
Address Resolution Protocol
For devices to communicate, the sending device needs the destination device’s IP address
and MAC address. When a device tries to communicate with a device whose IP addresses it
knows, it must determine the MAC addresses. The TCP/IP suite has a protocol called
Address Resolution Protocol (ARP) that can automatically obtain the MAC address.
ARP lets a computer find the MAC address of the computer that is associated with an
IP address, as shown in Figure 7-38.
Some devices keep ARP tables, which contain the MAC addresses and IP addresses
of other devices that are connected to the same LAN. ARP tables map IP addresses to
the corresponding MAC addresses. ARP tables are sections of RAM memory that are
maintained automatically on each device, as shown in Tables 7-10 and 7-11. It is rare
that you must manually make an ARP table entry. Each computer on a network main-
tains its own ARP table.
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