# 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.

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

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

(0000)

Range of Valid Hosts
(00011110)

(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.

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 Portable Command Guide
ISBN: 1587201585
EAN: 2147483647
Year: 2006
Pages: 261
Authors: Scott Empson

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