Understanding IP Subnetting: Subnet Mask Cheat Sheet

To be a successful network engineer or administrator, you’ll need to have a strong understanding of IP Subnetting. Networks don’t work without IP Addresses, and IP Subnets can make your networks more efficient – making your life easier in the long run.

In this article we’ll break down the basic facts about IP Subnetting to help give you some insight into the process. We also include a subnet mask cheat sheet for you to refer to in the future.

What is an IP address?

We can’t talk about IP Subnetting properly without first looking at IP Addresses. Most people have heard of an IP Address, but not many know what it actually is.

Every device that connects to the internet is allocated a unique IP (Internet Protocol) Address. This ensures data sent over the internet, such as messages or photos, reaches the right device. There are different versions of IP Addresses, but we’re focusing on IPv4 in this article.

In order to fully understand IP Addresses and Subnetting, it’s helpful to have a little binary mathematical knowledge.

IPv4 Addresses are 32-bit binary numbers, but for convenience, network engineers typically break these down into four 8-bit blocks instead.

So instead of looking like this: 11000000.10101000.00000001.01100100, an IP Address usually looks something like this: 192.168.1.100.

Every device will have its own unique IP Address in much the same way as every home or property has its own address.

What is a subnet?

An IP Subnet is a smaller network inside a larger network and its purpose is to make network routing as efficient as possible.

Networks are more effective when data can travel as directly as possible so subnets help to sort and route the data to ensure it takes an efficient course to its destination.

There are two parts to an IP Address – the network portion and the host portion. A subnet defines the number of bits, out of 32, used for the network portion of the address.

So, you might have a variety of devices including a computer, a printer, and a smart phone. Each device will have its own unique host number, but will be under the same network. For example:
Computer: 172.18.53.40
Printer: 172.18.53.55
Smart phone: 172.18.53.100

Notice that each device in the network has the same first three sections. That’s the network portion of this IP Address – also known as the subnet.

The last section is the host portion of the IP Address. That’s where you’d assign your individual devices. Systems within the same subnet can communicate directly with each other, while systems on different subnets must communicate through other means, such as a router.

What is a subnet mask?

A subnet mask is a number that defines a range of IP Addresses available within a network. Subnet masks hide the network portion of a system’s IP Address, and leaves only the host portion as the device identifier.

The class of subnet mask you use will depend on the size of your network.

Home networks usually use the default subnet mask of 255.255.255.0. An office network may use a different subnet mask such as 255.255.255.192 which allows up to 64 IP Addresses, while large networks with thousands of devices may use a subnet mask of 255.255.0.0, allowing more than 65,000 IP Addresses.

You can calculate IPv4 subnet addresses by hand if you want to, but it’s easier to download the IPv4 Cheat Sheet instead. This will help you optimise your network more quickly and easily.

Why should you utilise IP Subnetting?

There are several reasons why you should consider IP Subnetting. It will take a little planning and may be time consuming, but it’s definitely worth it. Here are a few examples of the benefits of subnetting.

• You can allocate limited IP Addresses more efficiently, helping to free up IPv4 Addresses.
• The improved efficiency will help to improve your network performance.
• You will ensure traffic destined for a device within your subnet stays in that subnet, therefore reducing network congestion.
• By splitting your network into subnets, you can control the flow of traffic allowing you to identify threats, close points of entry, and effectively make your network more secure.
• You can create networks that have logical host limits making your job as a network administrator far easier.

With the huge increase in demand for devices in this day and age, you can imagine the strain this is having on the IP Address Market. As a result we’re quickly running out of IPv4 Addresses, so this is where IP Subnetting really comes in handy.

For example, you may have a company with 100 computers. With Subnetting, rather than assigning 100 individual IP Addresses to your network, instead you can have a single IP Address for your network which has a certain number available for new devices.

So you can see how IP Subnetting can help to make your network more efficient.

We hope you found this article helpful and that it’s given you a better understanding of IP Subnetting. Don’t forget to download the IPv4 Cheat Sheet to make your life even easier.