Subnetting a Class C Network Using Binary | CCNA Portable Command Guide

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.

2^N 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.