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Like IP addressing, IP subnetting is also one of the most important topics for the CCENT
and CCNA R&S certifications. You need to know how subnetting works and how to “do
the math” to figure out issues when subnetting is in use, both in real life and on the exam.
Parts IV and VI of this book cover the details of subnetting concepts, motivation, and math,
but you should have a basic understanding of the concepts while reading the Ethernet topics
between here and Part IV.
Subnetting defines methods of further subdividing the IPv4 address space into groups that
are smaller than a single IP network. IP subnetting defines a flexible way for anyone to take a
single Class A, B, or C IP network and further subdivide it into even smaller groups of consecutive
IP addresses. In fact, the name subnet is just shorthand for subdivided network.
Then, in each location where you used to use an entire Class A, B, or C network, you can
use a smaller subnet, wasting fewer IP addresses.
To make it clear how an internetwork can use both classful IPv4 networks as well as subnets
of classful IPv4 networks, the next two figures show the same internetwork, one with
classful networks only and one with subnets only. Figure 4-8 shows the first such example,
which uses five Class B networks with no subnetting.
Figure 4-8 Example That Uses Five Class B Networks
The design in Figure 4-8 requires five groups of IP addresses, each of which is a Class B
network in this example. Specifically, the three LANs each use a single Class B network, and
the two serial links each use a Class B network.
Figure 4-8 wastes many IP addresses, because each Class B network has 216 – 2 host
addresses—far more than you will ever need for each LAN and WAN link. For example,
the Ethernet on the left uses an entire Class B network, which supports 65,534 IP addresses
that begin with 150.1. However, a single LAN seldom grows past a few hundred devices, so
many of the IP addresses in Class B network 184.108.40.206 would be wasted. Even more waste
occurs on the point-to-point serial links, which need only two IP addresses.
Figure 4-9 illustrates a more common design today, one that uses basic subnetting. As in the
previous figure, this figure needs five groups of addresses. However, in this case, the figure
uses five subnets of Class B network 220.127.116.11.
Figure 4-9 Using Subnets for the Same Design as the Previous Figure
Subnetting allows the network engineer for the TCP/IP internetwork to choose to use
a longer part of the addresses that must have the same value. Subnetting allows quite a
bit of flexibility, but Figure 4-9 shows one of the simplest forms of subnetting. In this case, each subnet includes the addresses that begin with the same value in the first three
octets, as follows:
■ One group of the 254 addresses that begin with 150.9.1
■ One group of the 254 addresses that begin with 150.9.2
■ One group of the 254 addresses that begin with 150.9.3
■ One group of the 254 addresses that begin with 150.9.4
■ One group of the 254 addresses that begin with 150.9.5
As a result of using subnetting, the network engineer has saved many IP addresses. First,
only a small part of Class B network 18.104.22.168 has been used so far. Each subnet has 254
addresses, which should be plenty of addresses for each LAN, and more than enough for
the WAN links.
In summary, you now know some of the details of IP addressing, with a focus on how it
relates to routing. Each host and router interface will have an IP address. However, the
IP addresses will not be randomly chosen but will instead be grouped together to aid the
routing process. The groups of addresses can be an entire Class A, B, or C network number
or it can be a subnet.