In this article I will show you a quick and easy way to work out the subnet mask from slash notation in 3 easy steps.
- Identify how many bits have been used. If you are presented with an IP address and it has a slash notation on the end -- e.g. /24 -- this is just another way of writing the subnet mask. It essentially means that you have used 24 bits to create the subnet mask.
- Translate slash notation to binary numbers. Once you know how many bits are being used you can now write out the slash notation in its binary equivalent. If we use the example above, we know that we have used 24 bits. Now that you know this, you will need to write the number out in groups of eight. Once you have reached the number of bits you have borrowed, fill the rest of the octets in with 0’s.
- Translate the binary numbers into a subnet mask. Now that you have the binary numbers, you can easily translate them into a subnet mask. You do this by taking each subnet in turn and converting it into the correct denary number. You can use the binary scale to help.
The first octet has all ones in it, so you place a one under each number in the binary scale. Once you have used all the ones up for that octet you add all the numbers above them to find out the denary number of that octet.
1 2 4 8 16 32 64 128
1 1 1 1 1 1 1 1
The denary number for the first octet would be 255. If you follow this for the rest of the binary numbers, you will get your subnet mask. In this case it would be 255.255.255.0.
This process might seem quite hard the first time you read through it if you do not have a lot of knowledge with binary numbers. But if you try it a couple of times, you will soon see how easy it really is to subnet.