Figuring out VLSM

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Trying to get my head around VLSM at the moment and was wondering if any of you guys could give me some advice.

I am attaching a spreadsheet of subnet addresses and was wondering if this was possible? The reason I'm asking is that the books say that you take the largest number of hosts first and work sequentially from there, but I have to fit a certain number of hosts within a specific subnet i.e the /32 subnet.

Hope you understand what I'm trying to get at.

 

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I am a Cisco Networking Academy Program Semester 4 graduate.

As you may know by now, a network that is subdivided is called a subordinate network or subnet for short. As you might recall, the procedure of creating subnets is to borrow host bits in the IP address to use as subnet bits. Well, think of VLSM as "subnetting a subnet." However, let's review basic subnetting.

To create a subnet, there are three types of requirements:

1. Minimum number of subnets required, with no required limit on hosts.
2. Minimum number of hosts required, with no required limit on subnets.
3. Minimum number of subnets required and minimum number of hosts required.

In your case study problem, VLSM subnetting is associated with case number 2 of "Minimum number of hosts required, with no required limit on subnets."

So step one is to list all of the "minimum number of hosts required" for each subnet in any order. Use column headings of "Department" and "Hosts required." Post back with your list.

 

No, it won't work as your spreadsheet has them laid out. The reason you have to go from largest to smallest is because you can only subnet an address range at certain locations. For example, let's take your subnet that requires 36 addresses. As you correctly determined, you'll need a subnet that can hold 62 hosts, so you use a 255.255.255.192 (/26) mask. This mask can be used to subdivide the 223.0.0.0 network into the following subnets:

223.0.0.0 to 223.0.0.63
223.0.0.64 to 223.0.0.127
223.0.0.128 to 223.0.0.191
223.0.0.192 to 223.0.0.255
...multiples of 64 (62 hosts... plus one network address, plus one broadcast address)

As you can see, your 223.0.0.40 subnet with a 255.255.255.192 mask won't work because 223.0.0.40 doesn't fall on one of those ranges. See what I'm gettin' at? So you'd want to take any of those ranges above. You can start at the end, you can start at the beginning, you can take one from the middle if you wanted to. But I'd recommend just taking the first subnet.

Let's take your next largest, which requires 18 addresses. As you accurately determined, you should use a 255.255.255.224 (/27) mask (for a maximum 30 hosts). This mask can be used to subdivide the 223.0.0.0 network into the following subnets:

223.0.0.0 to 223.0.0.31
223.0.0.32 to 223.0.0.63
223.0.0.64 to 223.0.0.95
223.0.0.96 to 223.0.0.127
223.0.0.128 to 223.0.0.159
223.0.0.160 to 223.0.0.191
223.0.0.192 to 223.0.0.223
223.0.0.224 to 223.0.0.255
...multiples of 32 (see how we get that? 30 hosts + 1 net + 1 broacast).

If you've already consumed 223.0.0.0 to 223.0.0.63 for R&D, where would you stick this Sales & Marketing subnet?

You should be able to tackle the rest from here.

 

Thanks for the replies guys!!

Sorry for wasting both your time but I should have read the question more carefully. Got it sorted out now.

Great explanation by the way BosonMichael.

 

Glad to be of assistance.

EDIT: Thanks for everyone who gave me praise in the rep fields. I appreciate it! Glad to help.