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Creating a Subnet

5/25/21  |  Eric Haff, Rexel Technical Consultant

If you are new to networking, it is easy to become overwhelmed by the seemingly endless acronyms and jargon that surrounds you. Do not worry, in time and with enough practice, you will learn to talk the talk and walk the walk. Today I would like to focus on the basic concept of a subnet to give you a better understanding of networking.

A subnet or subnetwork is a logic-driven subdivision of an IP network. When you divide a network into two or more networks, it’s called subnetting.

Finding the Subnet Mask

First, let’s look at IP addresses, the 32-bit unique identifier for a device on a network. People are used to seeing something like 192.168.0.10 and then that 255.255.255.0 in the box underneath. That 255.255.255.0 is the subnet mask. This value defines the network that the corresponding IP address is a part of by breaking down the octets into host and network bits. Sometimes you may see that subnet mask value displayed in CIDR block notation. That would look something like 192.168.0.10/24 for the example above. 

Breaking Down Bits

To understand what this means, we need to start by breaking down the numbers above into their bits and octets. 

Let’s take that example we had earlier:     192.168.0.10 

The example above in is dotted decimal notation. This is what it would look like if it were broken into its bit form: 11000000.10101000.00000000.00001010

Now, let’s do the same thing with our subnet mask:     255.255.255.0

This is what it would look like if it were broken into its octets and bits: 11111111.11111111.11111111.00000000

The green numbers are the host bits, and the red numbers are the network bits.

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Finding Key Network Information

Now that we have our bits for both our IP address and our subnet mask, we can determine four very important things about our network:

  1. Network Address
  2. Broadcast Address
  3. First Usable IP address
  4. Last Usable IP Address

You do this manually by using a process called bitwise, but for the purpose of this we will use a subnet calculator:

With this tool we can see that:

  • The Network Address or Subnet ID is 192.168.0.0
  • The Broadcast Address is 192.168.0.255
  • The First Usable Address is 192.168.0.1
  • The Last Usable Address is 192.168.0.254

Creating Smaller Networks

Any client that has an IP address between 192.168.0.1 and 192.168.0.254 with a 255.255.255.0 subnet mask will be able to communicate on the network without the need for a router or default gateway.

Let’s look at one more example that has a less widely used IP scheme:

In this example, we are using the same IP address 192.168.0.10, but we changed the subnet mask from 255.255.255.0 to 255.255.255.192

By doing this, we have created four smaller networks! The new range of the network that the address 192.168.0.10 sits on is 192.168.0.1-192.168.0.62. The new broadcast address is 192.168.0.63. And the network address stayed the same (192.168.0.0)! 

Managing Broadcast Traffic

So, what does this do for us and why is this important? Well, in networking a large portion of the protocols that manage our services are what is known as broadcast traffic. Broadcast traffic is sent to everyone on the network and can become an issue if not managed properly. One cool thing about broadcast traffic is that it does not traverse routers! Therefore, if you limit your subnet to a smaller number of hosts, you limit the amount of broadcast traffic those hosts have to deal with! Industrial automation equipment utilizes a host of broadcast traffic protocols. As we continue to grow the number of devices on our Industrial Automation and Control System (IACS) networks, we will need to manage this traffic and subnetting is the perfect tool. 

Are you interested in learning more about subnets? We can help. Contact us today!