# Subnetting For Fun and Profit

Networking and Communications

Oct 24, 2013 (5 years and 6 months ago)

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CCNA I

Module 10
-

Subnetting For Fun
and Profit

Why Subnet?

Subnetting allows you to limit

It also allows you to control access
between subnets.

How Subnets Work

router(s) how many bits
have been “borrowed” to
create the subnets.

An Example

Think of a class C network. A Class C network
might have an address of 168.192.1.0 with a

The first 3 octets are the network part of the
address. The last octet is for the hosts.

In this case you have one network that has 256
-
255 + 0), 254 of which can be
used. 168.192.1.0 is the network address.
of these can be used for hosts.

Now let’s Subnet

We will “borrow” three bits from
the last octet and use them as part

In binary the subnet mask would then
look like this:
11111111.11111111.11111111.11100000

In decimal it would look like this:
255.255.255.224

? We Borrowed 3 Bits

That left us with 5 bits in the
-
3=5)

2
5
-
2 for the
each subnet.

How many subnets?

To figure out the number of subnets just
look at the number of bits we borrowed.

Take 2 to the power of the number of bits
we borrowed. 2
3

= 8

Then subtract the first and last address
because they are the network and
-
2=6

There are 6 usable subnets with 30 host

Now let’s plot them:

Network
Number

Network

Usable host
range

0

192.168.1.0

1
-
30

192.168.1.31

1

192.168.1.32

33
-
62

192.168.1.63

2

192.168.1.64

65
-
94

192.168.1.95

3

192.168.1.96

97
-
126

192.168.1.127

4

192.168.1.128

129
-
158

192.168.1.159

5

192.168.1.160

161
-
190

192.168.1.191

6

192.168.1.192

193
-
222

192.168.1.223

7

192.168.1.224

225
-
254

192.168.1.255