How to create subnets
There are a couple of ways to create subnets. In this article we will subnet a class C address 192.168.0.0 that, by default, has 24 subnet bits and 8 host bits.
Before we start subnetting, we have to ask ourselves these two questions:
1. How many subnets do we need?
2x = number of subnets. x is the number of 1s in the subnet mask. With 1 subnet bit, we can have 21 or 2 subnets. With 2 bits, 22 or 4 subnets, with 3 bits, 23 or 8 subnets, etc.
2. How many hosts per subnet do we need?
2y – 2 = number of hosts per subnet. y is the number of 0s in the subnet mask.
An example will help you understand the subnetting concept. Let’s say that we need to subnet a class C address 192.168.0.0/24. We need two subnets with 50 hosts per subnet. Here is our calculation:
1. Since we need only two subnets, we need 21 subnet bits. In our case, this means that we will take one bit from the host part. Here is the calculation:
First, we have a class C address 192.168.0.0 with the subnet mask of 24. Let’s convert them to binary:
192.168.0.0 = 11000000.10101000.00000000.00000000
255.255.255.0 = 11111111.11111111.11111111.00000000
We need to take a single zero from the host part of the subnet mask. Here is our new subnet mask:
255.255.255.128 = 11111111.11111111.11111111.10000000
Remember, the ones in the subnet mask represent the network.
2. We need 50 hosts per subnet. Since we took one bit from the host part, we are left with seven bits for the hosts. Is it enough for 50 hosts? The formula to calculate the number of hosts is 2y – 2, with y representing the number of host bits. Since 27 – 2 is 126, we have more than enough bits for our hosts.
3. Our network will look like this:
192.168.0.0/25 – the first subnet has the subnet number of 192.168.0.0. The range of IP addresses in this subnet is 192.168.0.0 – 192.168.0.127.
192.168.0.128/25 – the second subnet has the subnet number of 192.168.0.128. The range of IP addresses in this subnet is 192.168.0.128 – 192.168.0.255.