Networking Basics: What Is IPv4 Subnetting?

First, let’s deal with the definition. Subnetting refers to the process involved in splitting a network into a smaller one. This is called subnets. This process is typically used to free up additional public IPv4 addresses as well as section networks for better management and security. To fully understand what subnetting is all about, it is crucial that you first grasp the binary and decimal structure of the IP address.

To explain better, we will begin with the basics. The IP address is something like this – 194.168.1.28. It is important to mention that the IPv4 address features a 32-bit number, and to make it more standard, they are further grouped into four 8-bit numbers, also known as octets. The octets are divided by decimal points. The number of octets range from 0-255. Therefore, the largest IP address that you can ever find is 255.255.255.255. In case you are wondering why they only get to 255 and not beyond, the simple answer is that they are in binary numbers. When converted to binary, the aforementioned IP address will look something like this – 11111111.11111111.11111111.11111111. If you look closely, you will notice that there are specifically eight numbers in-between the decimals. Each of the numbers indicates a bit, so it is called an 8-bit or octet grouping.

Definition of Network Segment of Subnet IP Address

In the early stages of the Internet adoption, the enterprises usually assign IP addresses to practically everything. Thankfully, the IP addressing designer has developed the perfect way to minimize the waste and has introduced subnetting that allows network sharing. As mentioned earlier, the process involved in splitting an extensive network into smaller bits is called subnetting and it helps to free up extra public IPv4 addresses. Basically, there are two sections for the IP address. These are the host portion and the network portion.

Definition of Subnet

A subnet refers to the number of bits that are used for the network portion from the 32-bit address. Additionally, the subnet masks may also be defined as a slash representation, which is also known as CIDR notation. Let’s take a look at an example to see how this works. The table below displays how bits are used within the network.

It is essential to note that the mask – 255.255.255.0 can be represented as /28. This is because it maintains 8-bits in portions of the total 32-bits that are used to explain the IPv4 address in the network portion.

Let’s look at another example. Let’s say you have a network with devices that have the IP addresses highlighted below:

Computer One: 175.16.56.40

Computer Two: 175.16.56.50

Network: 175.16.56.0

Printer One: 175.16.56.101

In the above scenario, the network uses three octets or 24-bits. Also note that the first three octets of each of the host devices within the network are the same. This is what is referred to as the network portion of the IP address with /24 mask. In this case, the IP address – 175.16.56.40; and the mask – 255.255.255.0; while the binary mask – 11111111.11111111.11111111.00000000. Here, the last octet in the formulation is the host section of the IP address. This is where you can assign your own devices. This means you can have as many as 254 hosts at the same time.

If you look at it again, you will find that if it was /16, this means that the first two octets in the formation will be the network portion while the host portion will be the last two sets of octets in the formation. That is, it will be something like this:

/16 – 255.255.0.0 – 11111111.11111111.00000000.00000000

Additionally, if it was /8-bits network, this means the first octet would be the only network portion. It will have the formation like this:

/8 – 255.0.0.0 – 11111111.00000000.00000000.00000000

It is essential to mention that the above are the most common types of masks and they are the simplest ones. However, when you require more than a single network, you will need to subnet. Subnetting usually allows you to select the number of bits that you require for a specific network portion. It is also possible to get additional bits from your host portion on the network.

Overview of IP Address Classes

Technically, there are five classes of IP addresses. However, only three of them (A, B, C) are appropriate for subnetting. Let’s look at the ranges of the IP address by their classes:

A: 1.0.0.0 – 127.0.0.0

B: 128.0.0.0 – 191.255.0.0

C: 192.0.0.0 – 223.255.255.0

It is crucial to note that the above IP addresses have been replaced with binary. The biggest subnet IP addresses within the ranges are highlighted below:

A: 127.0.0.0 – 01111110.00000000.00000000.00000000

B: 191.255.0.0 – 10111111.11111111.00000000.00000000

C: 223.255.255.0 – 11011111.11111111.11111111.00000000

It is very critical to understand this because it has an impact on the number of subnets and hosts that you can have in a single network. It is also essential to mention that you can take bits from each class to create additional subnets. However, you have to understand that you will be reducing the hosts’ number in the process.

How Can You Subnet in IPv4?

With a basic understanding of subnets, binary, and classes, it is time to take a look at a subnet. For this part, we’ll use an IP address like – 43.17.255.71/27. For this IP address, the binary will look like this – 00101011.000100011.111111.01000111. There are basically two things that are clear from this:

This is a class A IP address.
It needs to have a minimum of 8-bits in the network.

However, we will give it 27-bits. With this, the subnet’s network portion will look like this:

/27 – 255.255.255.224 – 11111111.11111111.11111111.11100000.

No doubt, subnetting requires the relevant skills and understanding. The good news is that if you are using a Cisco router that is already connected to a subnet, you can explore a more effective and easier method to get this done. Simply use the command – show IP route – in the Cisco router command line and the output will include the subnet for the specific IP address.

Conclusion

If you want to know more, you should understand that this is only the tip of the iceberg. To become an expert, you need to have not only theoretical but also practical knowledge. Don’t forget about it.