Custom Subnetting Question Class C:

So, I was watching the CBT Nuggets video and going through each example. I thought that I understood custom subnetting and then I came upon problem 3:


Network ID: 192.168.56.0
Minimum subnet hosts required: 12
Minimum subnets required: 13

Find...............

Custom Subnet Mask:
Maximum Hosts Per Subnet:
Maximum Subnets:
Host ID Ranges:


So, I work the problem before he goes over it in the video:

1. Custom Subnet Mask:

Take the number of of subnet hosts required, and convert it to a binary number. Count the number of binary digits. For this class, insert the number of digits in the customized octet, so that the custom subnet masks comes out looking like this:
11111111.11111111.11111111.11110000
255.255.255.240

Now, the problem seems to get a little easier.

2. Maximum hosts per subnet: shortcut: just count number of 0's on the custom subnet mask. Take the sum of the number of zeroes on the custom subnet mask and subtract two from that number. Take two to the power of custom subnet mask 0's less two = 2^4 - 2 = 14. Therefore, we have a maximum of 14 hosts per a subnet.

Again, everything seems to be going fine.

3. Next step Maximum Subnets: shortcut: count the number of ones in the octed that has been altered by the custom configuration. Take 2 to the power of that number, which is 2^4 in this case and you get 16. Therefore, we can have a maximum of 16 subnets. Again, everything looks fine.

4. Subnet ID's:
Start with the network ID as the first subnet. Go with multiples from there on the altered custom octet that correspond with the binary value of the last place filled in custom subnet masks.

192.168.56.0 +16
192.168.56.16 +16
192.168.56.32 +16
192.168.56.48 +16
and we continue the calculations until we have 16 subnets and the last number ending in 240.
At this point, I'm starting to see that custom subnetting is all about the figuring out the custom subnet masks.



Then, Suddenly, an abomination occurs. I realize that something is wrong with my host ID calculations, and I don't understand how to fix it. Can anyone help?

We start with the first host id range for the first subnet:
192.168.56.1 - 192.168.56.14

Next step: 192.168.56.15 (uh oh.. the binary value is all 1's: (11111111.11111111.11111111.1111)1111. And here is where I get confused. To get around the problem, the CBT Nuggets video recommends that we change the value to 192.168.56.16 so that it comes out like this 11111111.11111111.11111111.111(1)0000.

Now, this is where I'm confused. Here, you have just borrowed a number from a reserved place in the subnet mask (the fourth position) for the customized octet for a new ip address. It's been awhile since I took probabilities, but this all seems to be based on places. 2^4 made perfect sense for the number of hosts. 2^5 number of hosts which is what he does right here changes the value of the maximum hosts per subnet and the maximum subnets. In other words, it violates the conditions of the work set above.

So, here is a question: Can we really take 192.168.56.17 and use it in order to get around the all 1's issue. If so, why is this not a violation of the maximum hosts per a subnet and maximum subnets calculated above?

Someone please help me figure this out?

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