Subnetting a Class C Network Using Binary

Class C Network Subnetting Using Binary
Andrey Volkov

Andrey Volkov

System, network administration + DBA. And a little programmer!)) See Author profile.

You have an 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. You only work with host bits. You need to borrow some host bits and turn them into network bits (or in this case, subnetwork bits; I use the variable N to refer to both network and subnetwork bits).

Step 1. Determine how many H bits you need to borrow to create nine valid subnets.

2N ≥ 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 subnet in binary.

0000HHHH

 

00000000

All 0s in host portion = subnetwork number

00000001

First valid host number

00000010

Second valid host number

00000011

Third valid host number

 

00001110

Last valid host number

00001111

All 1s in host portion = broadcast number

Step 3. Convert binary to decimal.

00000000 = 0

Subnetwork number

00000001 = 1

First valid host number

00000010 = 2

Second valid host number

00000011 = 3

Third valid host number

. …

 

00001110 = 14

Last valid host number

00001111 = 15

All 1s in host portion = broadcast number

Step 4. Determine the second subnet in binary.

0001HHHH

 

00010000

All 0s in host portion = subnetwork number

00010001

First valid host number

00010010

Second valid host number

 

00011110

Last valid host number

00011111

All 1s in host portion = broadcast number

Step 5. 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 6. Create an IP plan table.

Subnet

Network Number

Range of Valid Hosts

Broadcast Number

1

0

1–14

15

2

16

17–30

31

3

32

33–46

47

Notice a pattern? Counting by 16.

Step 7. Verify the pattern in binary. (The third subnet in binary is used here.)

0010HHHH

Third subnet

00100000 = 32

Subnetwork number

00100001 = 33

First valid host number

00100010 = 34

Second valid host number

 

00101110 = 46

Last valid host number

00101111 = 47

Broadcast number

Step 8. Finish the IP plan table.

Subnet

Network Address (0000)

Range of Valid Hosts (0001–1110)

Broadcast Address (1111)

1 (0000)

192.168.100.0

192.168.100.1–192.168.100.14

192.168.100.15

2 (0001)

192.168.100.16

192.168.100.17–192.168.100.30

192.168.100.31

3 (0010)

192.168.100.32

192.168.100.33–192.168.100.46

192.168.100.47

4 (0011)

192.168.100.48

192.168.100.49–192.168.100.62

192.168.100.63

5 (0100)

192.168.100.64

192.168.100.65–192.168.100.78

192.168.100.79

6 (0101)

192.168.100.80

192.168.100.81–192.168.100.94

192.168.100.95

7 (0110)

192.168.100.96

192.168.100.97–192.168.100.110

192.168.100.111

8 (0111)

192.168.100.112

192.168.100.113–192.168.100.126

192.168.100.127

9 (1000)

192.168.100.128

192.168.100.129–192.168.100.142

192.168.100.143

10 (1001)

192.168.100.144

192.168.100.145–192.168.100.158

192.168.100.159

11 (1010)

192.168.100.160

192.168.100.161–192.168.100.174

192.168.100.175

12 (1011)

192.168.100.176

192.168.100.177–192.168.100.190

192.168.100.191

13 (1100)

192.168.100.192

192.168.100.193–192.168.100.206

192.168.100.207

14 (1101)

192.168.100.208

192.168.100.209–192.168.100.222

192.168.100.223

15 (1110)

192.168.100.224

192.168.100.225–192.168.100.238

192.168.100.239

16 (1111)

192.168.100.240

192.168.100.241–192.168.100.254

192.168.100.255

Quick Check

Always an even number

First valid host is always an odd #

Last valid host is always an even #

Always an odd number

Use any nine subnets—the rest are for future growth.

Step 9. Calculate the 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 network or a Class A network using exactly the same steps as for a Class C network; the only difference is that you start with more H bits.

 

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