What Is A Subnet Mask

mask imageSetting up your ip addresses is one of the first things you do when setting up a wireless home network.

Understanding what an ip address is fairly easy, it is simply a numbered address for your computer on the network. But thee are a few more details that you must know and understand. One of those is a subnet mask. And why do you need one?

In this article we're going to go over the subnet mask, why you need it and how this will effect your network.

Home routers are a bit different from your home computer in that they have two types of network interfaces. A WAN interface and one or more LAN interfaces. The only difference between these two is that the WAN interface connects to your Internet Service Provider (ISP) and the LAN interfaces are where you connect your home network.

Your WAN interface ip address settings many times are left to be auto configured by your ISP with something known as DHCP (Dynamic Host Control Protocol). This is where you ISP will assigned all of your WAN settings for you so that you can connect to their network

Your LAN settings can also be configured for DHCP so that anytime you connect your laptop, desktop, or game console to the router, the ip address settings are auto-magically configured for you.

To do this you have to start with the basics so that your computers function properly.

Understanding Your Network Settings

There are three fields of information that is needed for a home router to give to your home computer. You will need:

IP Address - this is the numerical address on the network for your computer.
Default Gateway - this is the ip address of the device (usually a router) that will send you off network and typically out to the internet.

Subnet Mask - this is the tricky one to understand.

Many times, these numbers can be very confusing.  To make things a bit easier, we have also provided a subnet calculator to help you figure out the proper subnet mask and ip address for your home network.

What is a Subnet Mask

  • The subnet mask is a numerical address that is used to separate the local network from other networks. (I told you this might be tricky)

What this means is that the subnet mask tells your computer and your router how large your home network is and when a request to an address that isn't included in this network needs to get sent to the default gateway to be routed to another network.

what is a subnet mask image

To understand this a bit more lets look at an IP Address itself. Lets use - this would be the ip address of your router. An ip address is broken down into what is known as 4 separate OCTETS. These are four different sets of number divided by a DOT or a PERIOD. They are called OCTECTs because they are a decimal representation of a binary numbers (8 bits). Hence the name OCTET.

Your laptop in the network might get an ip address of -

If we compare these two addresses we can see that the first 3 OCTETS are the same

If we compare these to the subnet mask we can

What the 255 in each of the first 3 OCTETS tells the computer that these numbers are NOT part of the local network.  So any ip address that begins with 192.168.0.x (x = any computer on your network) belongs to the local home network.  Anything else is another network and the router much send that data to the ISP.

So for example the ip address would get sent to the ISP, however would stay on your home network.

I hope this has all made sense.


Most people when they get a home router, they simply leave the network settings alone, keeping the defaults. This is fine, however this can put you in a vulnerable position with regards to network security.

Most home routers use a default network address of either or Most use the latter. Home routers also tend to use the very first ip address in a network, such as

Knowing what you now know, you might want to think about changing these addresses to something else, and example would be:

This way if a hacker sitting outside your home is trying to gain access to your home network will have a much harder time guessing what your home ip address is.

If you still have questions or are still asking yourself What is a Subnet Mask... please leave a comment below and I would love to help you further!

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