Subnetting a Class c address Example



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Subnetting a Class C Address Example
You have been tasked with producing an addressing scheme to support five networks (subnets) within your LAN. You will be using the private class C address 192.168.10.0 /24.
Step 1 – Determine how many network bits you will need to borrow from the available host bits.
Use the formula 2n where n >= the number of networks needed.
2n >= 5 n=3
We will actually have 8 networks even though we only need 5 because 23 = 8
Step 2 – Change the subnet mask to reflect the value for n in step 1. Since n=3 add replace the first 3 zeros in the last octet to ones.
The original subnet mask was 11111111.11111111.11111111.00000000 or 255.255.255.0
The new subnet mask is: 11111111.11111111.11111111.11100000 or 255.255.255.224

Step 3 – Determine how many hosts you will have available on each network
Use the formula 2n -2 where n = the number of 0’s in your subnet mask
25-2 = 30
Step 4 – Determine the new network addresses
Determine the decimal value of the last bit (the last 1) in the subnet mask. In our example the last 1 is in the 32 position so our networks will be in multiples of 32


128

64

32

16

8

4

2

1

1

1

1

0

0

0

0

0

Network:


1 – 192.168.10.0 5 – 192.168.10.128

2 – 192.168.10.32 6 – 192.168.10.160

3 – 192.168.10.64 7 – 192.168.10.192

4 – 192.168.10.96 8 – 192.168.10.224


Step 5 – Determine your valid host and broadcast addresses.
The broadcast address will always be the last address in the range or 1 bit less than the next network.


IP Address

First Host Address

Last Host Address

Broadcast Address

192.168.10.0

192.168.10.1

192.168.10.30

192.168.10.31

192.168.10.32

192.168.10.33

192.168.10.62

192.168.10.63

192.168.10.64

192.168.10.65

192.168.10.94

192.168.10.95

192.168.10.96

192.168.10.97

192.168.10.126

192.168.10.127

192.168.10.128

192.168.10.129

192.168.10.158

192.168.10.159

192.168.10.160

192.168.10.161

192.168.10.190

192.168.10.191

192.168.10.192

192.168.10.193

192.168.10.222

192.168.10.223

192.168.10.224

192.168.10.225

192.168.10.254

192.168.10.255

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