Learning to subnet can often cause a challenge for engineers that are new to networking.  Understanding why we use it and the benefits of subnetting can be easy to grasp but knowing how to successfully design and implement can be a challenge.

In this blog I want to cover the basics of why we would want to subnet and talk about common terminology’s the we may come across.

Subnetting what is it and why do we want to use it?

Subnetting is a way of being able to make better use of our IPv4 addressing scheme by making smaller networks at a classful boundary.  Also it helps create logical boundary’s within out network to help segment network traffic.

So why do I want to do this?

Let me start but talking about using subnetting to creating logical boundaries to help segment traffic.  If we take medium to large networks, we could be talking about hundreds if not thousands of devices. When I refer to devices or hosts I mean PCs, Laptops, Servers, mobile devices.  These devices if placed in one flat logical network (or Subnet) can generate lots of traffic within the network.  A network must learn where the devices are connected, when a device does not know where a destination device is within the network the device sends out a broadcast and wait to see if the device will respond to the sender to determine where the device is.  This would be like you walking in to a room of a 1000 people and shouting out to everyone “do you know were bill is?”  all 1000 people will hear the request.  As we can see this would be a lot traffic as all 1000 devices on the network would hear this broadcast as it is sent to everyone.

Now what if I split the room in to 4 rooms of 250 and ask in the room where bill is again?  Only the 250 people in that room will hear this therefore generating only 250 request instead of 1000.  How does this fit in to subnets?

We have 4 departments:

  • Sales
  • Finance
  • Marketing
  • IT

These for departments are separated in to their own rooms or in our case subnets.  Each subnet restricts the amount of noise or broadcasts within its own subnet therefore reducing the amount of traffic on the network.  Subnets/networks can still talk to each other by using routing.

If we are using routing and our subnets can route to each other we put security in place with Access Control lists.  This give you the flexibility to allow department that are allowed to communicate speak to each other and restrict department to sensitive departments.  I sure there will be an internal audit if a employee was able to access the salary spreadsheet in finance and increase the amount they were earning!!!!


So to review

  • Subnets can be used to create smaller broadcast domains creating less traffic.
  • Subnets can talk still reach each other using routing within the network.
  • Security can be put in place with access control list to controls the flow of traffic within the network. This helps create security boundaries between departments.

In the next part I will explain about classful networks and show you how to subnet to make a network smaller.