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.

Subnetting IPv4… I think

Until recently, I didn’t really grasp the importance or relevance of subnetting.  After reading this week’s chapter, and watching some videos on YouTube, I realized how it works now and I have a larger understanding of how the IP is restricted based on the subnet (or at least I think I do).  

I had always assumed that the subnet was “another” IP address that you could create many networks from within.  But after this chapter, it has made it more clear to me that the subnet is more of a “locking” mechanism for network addressing.

I get the binary approach, in fact, breaking binary has always been easy for me, it was just the application to subnetting that was confusing.  I think the easiest way for me to remember this now is that the subnet locks the other digits so that your ranges are better defined…

My example is a home network:

IP Address 192.168.1.32/27 or 

1100000.10101000.00000001.00100000

The /27 denotes subnet is locked at 27 characters, so

11111111.11111111.11111111.11100000 or 255.255.255.224

lining them up helped me determine a few things:

                              |
                              V
11000000.10101000.00000001.001 00000  (ip)
11111111.11111111.11111111.111 00000  (subnet)
------------/27---------------^
                              |

Everything to the left of the break can’t change because of the subnet.  So, to find the basic information like network address and broadcast address is as simple as finding the top range and bottom range of the IP. 

Since, in this case, the first 3 digits can’t change in the last octet, the address, you just have to work the numbers that can change:

00100000 (or .32) = Network address

00111111 (or .64) = Broadcast address

That would leave you with 192.168.1.33 through 192.168.1.63 for usable addresses.