But what is a SUBNET MASK? - Short & Simple

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Subnet masks and subnetworking in general is a network engineer's must-know skill. It is also a skill which may come in handy for everyone.

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To understand subnet masks we must first understand what an IP address consists of.
An IP is a 32 bit address divided into 4 parts, where each part consists of 8 bits.
The IP also consists of what is called a network identifier and a host identifier.
The network identifer specifies the main network number, and this number is always the same on every device throughout the network.
The host identifier uniquely identifies each individual device on the network, and must always be different from device to device.
So a couple of computers under the same network, will always have the same network identifier, but they will never have the same host identifier.

But how do we know which part of the IP is the network ID and which part is the host ID?

This is when the subnet mask comes in.
The subnet mask is assigned to each IP and is what decides which part of the IP address is the network identifier and which part is the host identifier.
And just like the IP address, the subnet mask is also 32 bit long and is divided into 4 parts.
But it can only have 2 types of numbers: either 255 or 0.
255 is what declares the network identifier, and 0 is what declares the host identifer.
The /16 at the end means that the first 16 bits of the IP is assigned as the network identifier, aka the first 2 parts of the IP address as these parts are 8 bits each.
And so in this example 192.168. is the network identifer, while 13.37 is the host identifier.

This is thus a class B type of network since the subnet mask consists of 255.255.0.0.
A class A network would have 255.0.0.0/8, while a class C network would have 255.255.255.0/24 as a subnet mask.

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