Subnetting a Class C Network Using Binary


You have a Class C address of 192.168.100.0 /24. You need nine subnets. What is the IP plan of network numbers, broadcast numbers, and valid host numbers? What is the subnet mask needed for this plan?

You cannot use N bits, only H bits. Therefore, ignore 192.168.100. These numbers cannot change.

Step 1.

Determine how many H bits you need to borrow to create nine valid subnets.

2N 2 9

N = 4, so you need to borrow 4 H bits and turn them into N bits.

Start with 8 H bits

HHHHHHHH

Borrow 4 bits

NNNNHHHH


Step 2.

Determine the first valid subnet in binary.

0001HHHH

Cannot use subnet 0000 because it is invalid. Therefore, you must start with the bit pattern of 0001

00010000

All 0s in host portion = subnetwork number

00010001

First valid host number

.

 

.

 

.

 

00011110

Last valid host number

00011111

All 1s in host portion = broadcast number


Step 3.

Convert binary to decimal.

00010000 = 16

Subnetwork number

00010001 = 17

First valid host number

.

 

.

 

.

 

00011110 = 30

Last valid host number

00011111 = 31

All 1s in host portion = broadcast number


Step 4.

Determine the second valid subnet in binary.

0010HHHH

0010 = 2 in binary = second valid subnet

00100000

All 0s in host portion = subnetwork number

00100001

First valid host number

.

 

.

 

.

 

00101110

Last valid host number

00101111

All 1s in host portion = broadcast number


Step 5.

Convert binary to decimal.

00100000 = 32

Subnetwork number

00100001 = 33

First valid host number

.

 

.

 

.

 

00101110 = 46

Last valid host number

00101111 = 47

All 1s in host portion = broadcast number


Step 6.

Create IP plan table.

Valid Subnet

Network Number

Range of Valid Hosts

Broadcast Number

1

16

1730

31

2

32

3346

47

3

48

4962

63


Notice a pattern? Counting by 16.

Step 7.

Verify pattern in binary (third valid subnet in binary used here).

0011HHHH

Third valid subnet

00110000 = 48

Subnetwork number

00110001 = 49

First valid host number

.

 

.

 

.

 

00111110 = 62

Last valid host number

00111111 = 63

Broadcast number


Step 8.

Finish IP plan table.

Subnet

Network Address
(0000)

Range of Valid Hosts
(00011110)

Broadcast Address
(1111)

0 (0000) invalid

192.168.100.0

192.168.100.1192.168.100.14

192.168.100.15

1 (0001)

192.168.100.16

192.168.100.17192.168.100.30

192.168.100.31

2 (0010)

192.168.100.32

192.168.100.33192.168.100.46

192.168.100.47

3 (0011)

192.168.100.48

192.168.100.49192.168.100.62

192.168.100.63

4 (0100)

192.168.100.64

192.168.100.65192.168.100.78

192.168.100.79

5 (0101)

192.168.100.80

192.168.100.81192.168.100.94

192.168.100.95

6 (0110)

192.168.100.96

192.168.100.97192.168.100.110

192.168.100.111

7 (0111)

192.168.100.112

192.168.100.113192.168.100.126

192.168.100.127

8 (1000)

192.168.100.128

192.168.100.129192.168.100.142

192.168.100.143

9 (1001)

192.168.100.144

192.168.100.145192.168.100.158

192.168.100.159

10 (1010)

192.168.100.160

192.168.100.161192.168.100.174

192.168.100.175

11 (1011)

192.168.100.176

192.168.100.177192.168.100.190

192.168.100.191

12 (1100)

192.168.100.192

192.168.100.193192.168.100.206

192.168.100.207

13 (1101)

192.168.100.208

192.168.100.209192.168.100.222

192.168.100.223

14 (1110)

192.168.100.224

192.168.100.225192.168.100.238

192.168.100.239

15 (1111) invalid

192.168.100.240

192.168.100.241192.168.100.254

192.168.100.255

Quick Check

Always an even number

First valid host is always an odd #

Last valid host is always even #

Always an odd number


Use any nine subnetsthe rest are for future growth.

Step 9.

Calculate subnet mask.

The default subnet mask for a Class C network is as follows:

Decimal

Binary

255.255.255.0

11111111.11111111.11111111.00000000


1 = Network or subnetwork bit

0 = Host bit

You borrowed 4 bits; therefore, the new subnet mask is the following:

11111111.11111111.11111111.11110000

255.255.255.240


Note:

You subnet a Class B or a Class A network with exactly the same steps as for a Class C network; the only difference is that you start with more H bits.




CCNA Self-Study(c) CCNA Portable Command Guide
CCNA Portable Command Guide
ISBN: 1587201585
EAN: 2147483647
Year: 2006
Pages: 261
Authors: Scott Empson

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