How many subnets     

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If you are asked to work out a subnet mask that will supply a certain amount of subnets this is how you can do it.

Before you go any further it is best if you know how to convert binary in to decimal

In this example you are asked to create a mask for 10 subnets. This is quite easy as long as you understand the power of 2

128 64 32 16 8 4 2 1
2^7 2^6 2^5 2^4 2^3 2^2 2^1 2^0


So 2^ of what? makes the desired subnets, look at my chart above we have been asked to create a subnet that will cater for 10 subnets. So if we look at 2^3 this would give us 8 subnets this is not enough. So if we look at 2^4 this would give us 16 subnets this is plenty as we need 10.

So lets convert that to binary 2^4 so we are using 4 zero's and 4 one's

11110000 This binary number converts to 240 which is the last subnet in the mask.

You can look at my binary to decimal conversion which can be found here which will help more as you should be able to do that before you get to this stage where we are working out subnets

128 64 32 16 8 4 2 1
1 1 1 1 0 0 0 0