N = Network bits
H = Host bits
All 0s in host portion = Network or sub-network address
All 1s in host portion = Broadcast address
Combination of 1s and 0s in host portion = Valid host address
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.
Subnetting a Class B Network Using Binary
You have a Class B address of 172.16.0.0 /16. 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 172.16. 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.
Question 2
What is the network number of the IP address 192.168.100.115 if it has a subnet mask of
255.255.255.192?
What answer do you think you will get? Let’s find out!)
Answer
Step 1 Convert both the IP address and the subnet mask to binary:
192.168.100.115 = 11000000.10101000.01100100.01110011
255.255.255.192 = 11111111.11111111.11111111.11000000
Step 2 Perform the AND operation to each pair of bits—1 bit from the address AND to the
corresponding bit in the subnet mask. Refer to the truth table for the possible outcomes:
192.168.100.115 = 11000000.10101000.01100100.01110011
255.255.255.192 = 11111111.11111111.11111111.11000000
AND
result = 11000000.10101000.01100100.01000000
Step 3 Convert the answer back into decimal:
11000000.10101000.01100100.01110000 = 192.168.100.64
The IP address 192.168.100.115 belongs to the 192.168.100.64 network when a mask of
255.255.255.192 is used.