Create subnets

There are a couple of ways to create subnets. In this article we will subnet a class C address 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.

Subnetting example

An example will help you understand the subnetting concept. Let’s say that we need to subnet a class C address 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 with the subnet mask of 24. Let’s convert them to binary: = 11000000.10101000.00000000.00000000 = 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: = 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: – the first subnet has the subnet number of The range of IP addresses in this subnet is – – the second subnet has the subnet number of The range of IP addresses in this subnet is –

Prerequisites for 200-301

200-301 is a single exam, consisting of about 120 questions. It covers a wide range of topics, such as routing and switching, security, wireless networking, and even some programming concepts. As with other Cisco certifications, you can take it at any of the Pearson VUE certification centers.

The recommended training program that can be taken at a Cisco academy is called Implementing and Administering Cisco Solutions (CCNA). The successful completion of a training course will get you a training badge.

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