Subnetting For Fun and Profit

pigeoneggtrainsNetworking and Communications

Oct 24, 2013 (3 years and 11 months ago)

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CCNA I

Module 10
-

Subnetting For Fun
and Profit

Why Subnet?


Subnetting allows you to limit
broadcast domains to cut traffic.


It also allows you to control access
between subnets.

How Subnets Work


The subnet mask tells the
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
subnet mask of 255.255.255.0 .


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
addresses (1
-
255 + 0), 254 of which can be
used. 168.192.1.0 is the network address.
168.192.1.255 is the broadcast address. Neither
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
of the binary address.


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
host address. (8
-
3=5)


2
5
= 32 addresses
-
2 for the
network address and broadcast
address = 30 usable addresses in
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
broadcast address. 8
-
2=6


There are 6 usable subnets with 30 host
addresses in each.

Now let’s plot them:

Network
Number

Network
Address

Usable host
range

Broadcast
Address

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

Please Read Module 10!

We’re Running Way Behind.

Tests are already active.

There is a fair amount of detail
to go over!