Clarification on Class C Subnetting

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Please bear with me as you read through my thought process. I am working on a practice problem and I am trying to figure out why the numbers are not working out, or at least they don't appear to be.

The problem reads as follows:
I am given the 192.168.0.0/24 network and I need to subnet it to allow for 100 Hosts in the Staff Network, 50 Hosts in the Sales Network, 50 Hosts in the IT Network, and 25 Hosts in a FUTURE Guest Network.

I know from studying VLSM, that in order to come up with 100 hosts, I need 7 host bits. If I want to come up with 50 hosts, I need 6 host bits. For 25 hosts, I need 5 host bits.

I made the following in Word to help me visualize the process.

I know that I need cannot use the Network and Broadcast addresses in a particular subnet. Also, there will be an SVI on each of the three switches S1, S2, and S3. Router R1 will be the default gateway for each subnet.

Here is where my confusion comes into play. I know that I need a total of 225 host IP addresses, but using the VLSM, I come up with a total of 126+62+62+30 = 280 hosts, which is more than the 255 allotted in the fourth octet.

I'm not entirely sure if I am looking at this correctly? With the first three VLSM, I have a total of 250 hosts, that only leaves 5 leftover, which is not enough to cover the Guess Network of 25 hosts.

What is the correct way of thinking about the VLSM in this case?

Thank you,

Chris

@Bedrock1977,

Your issue lies in your 3rd VLSM subnet: 192.168.0.192 /26

You come up with a range of : 192.168.0.192 - 192.168.0.223 = 62 hosts????

This is actually /27 = 30 hosts...not 62 hosts.

Given your parameters...You CANNOT have from the given network 100, 62, 62, and future for 25 minutes.

You could do a 100, 62, 30 and 30 though...as that is how your VLSM math works out!

@Ronnie-Wong

Hi Ronnie,

Is there a rule someplace stating that I must stay within the 192.168.0.0/24 network or if I can extend into the 192.168.1.0 network?

I worked this particular solution, and it seems to work, but ONLY if I can move into the 1.0 network.

100 Hosts = 192.168.0.0/25
50 Hosts = 192.168.0.128/26
50 Hosts = 192.168.0.192/26
25 Hosts = 192.168.1.0/27

@Bedrock1977

My word! I think I completely misread the problem. The IT Department only needed 25 Hosts. So we have 100 Hosts in the Staff Network, 50 in the Sales Network, 25 in the IT Network, and 25 in the Guest Network. That works with what you said earlier; 100, 62, 30, and 30.

I apologize for not giving you the correct information upfront.

Chris