Nailing VLSM

[turbo_engine26]

Hello,

I am preparing for the CCIE R&S lab and have couple of doubts around VLSM. Subnetting itself is an easy task when you take a major network and divide it into several subnets. But this is currently not my problem. My problem is with VLSM or variable length subnet mask. I understand that VLSM is used to tailor your data links in order to accommodate certain number of host addresses, No doubt in that. However, i can't understand a point in the case study mentioned in the book that i am studying from.

Case Study: Using VLSM

The subnet 172.25.150.0/24 has been assigned to the internet. That subnet has been further subnetted to fit the various data links by expanding the mask to 28 bits (/28). Below are the available sub-subnets:

172.25.150.0/28

172.25.150.16/28

172.25.150.32/28

172.25.150.48/28

172.25.150.64/28

172.25.150.80/28

172.25.150.96/28

172.25.150.112/28

172.25.150.128/28

172.25.150.144/28

172.25.150.160/28

172.25.150.176/28

172.25.150.192/28

172.25.150.208/28

172.25.150.224/28

172.25.150.240/28

Now, we want to assign 60 hosts to one of the data links so the answer will be using /26 mask. Easy! - But which subnet should be used?

According to the book again, four subnets from the above can be combined into a single subnet with /26 mask and they are:

172.25.150.64/28

172.25.150.80/28

172.25.150.96/28

172.25.150.112/28

COMBINED TO:

172.25.150.64/26

My question:

Why specifically the .64 subnet is selected ? Why wasn't the .192 subnet for example selected?

I appreciate your valuable response.

Thanks

[Jon Marshall]

Why specifically the .64 subnet is selected ? Why wasn't the .192 subnet for example selected?

Hmmm, or why not 172.25.150.0/26 or 172.25.150.128/26 ?

Is that all the book said. And did it say that .64/26 was the only choice ? 

Jon

[turbo_engine26]

Thanks for the reply.

Well, there is a note at the end says "Note that the four subnets are not selected at random; the first 26 masked bits are identical and are unique within the group of 16 subnets".

Because /26 will give me 64 block size so i have 4 subnets: .0 (ip subnet-zero) , .64, .128, .192

In the book, the .0 , .128 are used already with /28 and i think it doesn't make sense to use them again with /26

So, we have .64 and .192.

Thansk for your help

[Jon Marshall]

Well, there is a note at the end says "Note that the four subnets are not selected at random; the first 26 masked bits are identical and are unique within the group of 16 subnets"

But this also applies to .0/.128 * 192.

I think what they are trying to say is you cannot for example choose 172.25.150.32/26 which wouldn't work.

Jon

[turbo_engine26]

Yes i understand that, since the 64 block size increments only by 64 , there is no way to consider 16 or 32 or 48 which are less than 64

I think i found something so correct me if i am wrong.

The below networks are in use as well:

.224/28

.240/28 (used for serial links with /30 mask)

I think we can't use the .192 as this is the last subnet if we use 64 block size. In others words, we cannot use .192 and .256 .. Make sense now!

So again,

.0 is used

.128 is used

.64 not used

.192 can't be used

Thats' why the author chose .64? ... am i right?

[Richard Burts]

Unfortunately no you are not right. You seem to be saying that 192 can not be used because it is the last subnet but that is not a real restriction. 192 could be used very well.

Based on what you have told us, either the 64 or the 192 could have been used. Choose one or the other. My guess is that the book chose 64 because it came first.

HTH

Rick

[edit] Many people take the restriction of "can not use the first or last address" which is absolutely true for addresses within a subnet, and try to apply the same rule to subnets within a network. It sort of works for the very first subnet (the very old rule of do not use subnet zero). But there never has been any restriction about using the last subnet. There is no such thing as a broadcast subnet.

[turbo_engine26]

Thanks Rick

So, according to your saying , if we use the .192/26 as a combined subnet instead then the four subnets will be:

.192/28

.208/28

.224/28

.240/28

But because, the 224 and 240 are used already so we cannot use this range?  ... I really don't understand why the book used the term combined subnets into one subnet to fit 60 host addresses to a link

Please let me know if this discussion is clear enough or you want me to post the whole case study with diagram

I am working on creating a diagram for this and post it.

[Jon Marshall]

Yes you can use .192 with the subnets you have used.

I think you may be attaching too much importance to this. I suspect they simply used .64 because they hadn't used it anywhere else and it is the first available. Nothing more important than that.

Jon

[Richard Burts · Accepted solution]

My apologies for not reading thoroughly. My response was addressing mostly your comment that 192 can not be used because it is the last subnet. And that is not a reason for not using 192. But I did not pay enough attention to where you mention that 224 and 240 are already used for other purposes. Given that information then it is correct to say that 192 can not be used. And therefore 64 is really the only viable choice.

Now I would like to address this statement:

I really don't understand why the book used the term combined subnets into one subnet to fit 60 host addresses to a link

The book started with the assumption that you had a network that was /24 and would be subnetted into subnets using mask /28. They created 16 subnets where each subnet contains 16 addresses (and 14 useable host addresses). Note that since all subnets use the same mask that this is an example of FLSM (Fixed Length Subnet Mask) and is the original/old approach to subnetting.

Then the book introduced a requirement that there should be a subnet capable of 60 addresses. This moves the discussion into VLAM (Variable Length Subnet Mask). They want to create a single subnet with 60 host addresses. To do this they take 4 of the original /28 subnets and combine them into a single subnet with mask /26 and that does support 60 addresses.

HTH

Rick

[turbo_engine26]

Exactly! ... Hmmm, now i understand , thanks a lot

[turbo_engine26]

I think you may be attaching too much importance to this

When it comes to designing networks and avoiding routing problems, then everything is important to consider.

Whe  i posted the discussion i mentioned the word "doubts" which means that i  understand the topic of VLSM but came here for discussion to break  those doubts and to correct me if i am wrong. All i wanted to know is  why the .192 subnet were not used?

I inserted the diagram here. Don't know why i can't see the upload attachment area anymore. Anyways, you will notice that the  .192 subnet is not used at all. However, because the .224 and .240 are  used one for Ethernet link and one for serial link, maybe this is the  reason why .192 is not used.

I know it sounds like i  am posting a question and answer myself BUT, the most important to me is  to judge if that answer is right or wrong.

Thanks a lot

[Jon Marshall · Accepted solution]

Apologies if my wording caused any offence. It wasn't meant to, rather it was mean to reassure you that you did indeed understand the issues and that it was the book that was milseading and not your understanding.

That said, you didn't make it clear in your original question that a lot of the /28 subnets were already in use so we didn't have the full picture.

I agree with what you say. Some of the /28 subnets after .192 are already in use so it would seem that is why they chose to use .64/26.

Jon

[turbo_engine26]

No harm done    ... Also, i am not offended at all .. In my point of view, discussion means positive and negative thoughts. Negative thoughts should be respected by the creator of the post. Because i created this post, it doesn't mean to expect only positive thoughts that agree with me or i consider this as Selfishness from my side. Also, having people at least reply to me is a big appreciation in its own.

As Rick said down there, i beleive that i nailed the topic and i can go for troubleshooting section in the book with confidence.

Thanks Jon and Rick for your kind participations in this post and your thoughts are always respected.

[Richard Burts]

I believe that you have now nailed the explanation. If the 224/28 and 240/28 subnets are already assigned then 192/26 is not available for use.

HTH

Rick