How to subnet an IP address the easy way
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If you have ever heard networking geeks talk about subnetting or you are trying to learn it on your own, you may be wondering how anyone does all that subnetting magic binary math. Learning the math behind the subnetting is the key to a solid foundation. However, if you need to do subnetting really quickly (like for a certification exam), the following steps might help.
Most of the information for a subnetting question can be found by using the binary chart:
128 | 64 | 32 | 16 | 8 | 4 | 2 | 1 |
128 | 192 | 224 | 240 | 248 | 252 | 254 | 255 |
The top of the chart is binary through 128. The bottom of the chart are the subnet mask values. (If you will notice, you get these values by first bringing down the 128, and then adding the next binary value to it e.g. 128+64=192, 192+32=224, 224+16=240 and so on.) Now here is the trick to using this chart:
Say you have a subnet mask of 255.255.248.0 Well, from this mask you know that the subnetting will be done in the third octet.
-Fill in 1’s in the chart until you reach 248 on the third octet from left to right. (That would be 8 ones in the first row, 8 ones in the second row, and 5 ones in the third row.) In CIDR notation this would be /21.
Now you know that this is a /21 subnet mask.
-Just do the same process if you do not know the decimal subnet and you do know how many bits you have borrowed. So if you had to borrow 5 bits from a class B address, then you would end up with a /21 (just fill in the 1’s).
Now comes the cool part. Finding your network increment. (In other words, knowing on which address the network begins and ends.)
-Having just filled in the 1’s on your chart you should see that you have stopped filling in 1’s at the 248 column in the third row of the binary chart.
-Now follow that column up until you get to the top row where the 8 is in the binary chart. Guess what? 8 is your network increment.
So you know that the Network ID’s are going to be multiples of 8. (*.*.8.*, *.*.16.*, *.*.24.* and so on.) This method works every time. It is a handy shortcut that has helped me on many networking tests. I hope that it helps you as well. Good luck with your certifications or other subnetting endeavors.
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