The subnet mask is an intricate part of your network connection configuration. If you’ve ever needed to set a static IP address for any reason you’ve probably wondered what to put in the subnet mask field. The subnet mask is what tells your machine which part of your IP address is the network portion and which part is the host portion. To explain this best let me quickly go through what an IP address is made up of.

The IP address – An IP version 4 address is 32bits long and split into 4 octets (four 8 bit sections), for example: 22.56.90.189.  Imagine that each bit of the address can either be a 1 or a 0 and then at each octet can be from 0 (all off) to 255 (all on). Check out the below for a visualisation.

This particular block would be a 0 because all the bits are turned off. If we turned the first bit on (to a 1) then this block would be 128. This is the binary mathematics behind an IP address and the same principle applies to the subnet mask. Remember I said about network and host portions? This is basically to identify what network you belong to. Consider the following IP address and subnet mask:

IP Address – 192.168.1.55
Subnet Mask – 255.255.255.0

The way that the subnet mask works is that all bits that are turned on are the network bits, and all bits turned off are the host bits. In this example we had to turn all bits on in the first 3 octets to give us 255.255.255. The 0 block obviously has no bits turned on and this is the host block. So this would mean that our network is 192.168.168.x or 192.168.168.0. This means that anything in the fourth octet is a host, apart from 2 exceptions which I’m going to talk about!

The network address: The network is used to represent all hosts in a network; it is used by routing tables to make decisions based on individual networks as opposed to individual hosts. In a standard subnet mask like 255.255.255.0 the network id would be 0 because it is the first number within the network (remember we go from 0 to 255). So in this example our network address is 192.168.1.0

The broadcast address: This is the address that is reserved to send a ‘broadcast’ to all hosts within a network. It is a special address and is the last usable address in a network, so in this example the broadcast address would be 192.168.1.255.

Unless you are subnetting for any reason then your subnet mask is going to be standard, and if you are using a private IP address of 192.168.something.something then your mask should be 255.255.255.0. If you were using a class b private address (172.16.something.something – 172.31.something.something) then your mask should be 255.255.0.0. Finally if you were using a class A address (10.something.something.something) then your mask should be 255.0.0.0.

Subnetting – I could devote several pages to subnetting as it is indeed a tricky thing to get to grips with. What subnetting does is split a network into two or more smaller networks. When you do this your subnet mask will no longer be standard. Unless you have any special requirements in your home network you shouldn’t really need to subnet.

If you want to view your subnet mask then click start and run (or just type in the search box) cmd and click ok. In the command prompt type ipconfig and press enter. You will be shown your ip address, subnet mask and default gateway.