Subnet Cheat-sheet

Using the cheat-sheet has really helped cut back on the binary conversion for subnetting. I work in a unique environment where people deal with subnetting all of the time and one of my colleagues showed me a simple way to calculate IP ranges and subnets by using this. It seems like a lot at first, but once you use it a few times, it really becomes simple and makes sense.

      128   192   224   240   248   252   254   255  (Subnet)
      128    64    32    16    8     4     2     1   (Binary)
1 A    1     2     3     4     5     6     7     8   <------
2 B    9    10    11    12    13    14    15    16   CIDR
3 C    17   18    19    20    21    22    23    24
4 D    25   26    27    28    29    30    31    32

So, if I was to look at a random address:  6.75.132.4/26

In order to use the chart above, I first have to look at the CIDR.  Find the corresponding top number from the CIDR area in the sheet above.  Since it’s /26, it’s in the last row, so that tells you it’s a D class network.  Following the same logic, find the top number and it’s 192.  So this means that your subnet is 255.255.255.192.

Now that you have the subnet, look at the second row.  The value is 64 (which is what you get when you follow /26 up to the second row).  That tells you that your network ranges count by 64.  So, you look see what range the provided address is in… the last octet is .4, so it falls within 0-63 (which is 64 numbers).  Now you have your first two numbers… 0 and 63.  That is the Network Address and the Broadcast.

Finding the range is the easy part… just add one to the Network address and subtract one from the broadcast.  That is 1-62.

So, 6.75.132.4/26 is a host address within the following:

N (Network Address) – 6.75.132.0

F (First Host) – 6.75.132.1

L (Last Host) – 6.75.132.62

B (Broadcast) 6.75.132.63

Again, hopefully, this doesn’t confuse anyone more than it helps.  Once you practice this a few times, it really starts to make better sense.

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