How to add VLSM to an existing VLSM

[Fangerino]

Hi, Can someone teach me how to extend my VLSM (I'm not sure is it the correct way to say it).

Please refer to the attached, lets say my company decided to have 1 more dept and I was tasked to give them a network how should i go about it from /30 ?

[Fangerino]

Yes Please, the only thing I know about VLSM is Bigger Host come first follow by smaller 1 then u do the subnetting from there I know there's more to it but I just thought what if there is a way to continue subnetting from /30 but since like that is no such thing. Then I think I will need to do a new VLSM table and create on a different packet tracer and present from there.

[pieterh]

I formatted your table another way
this may help you understand the variable length in VLSM  and how space is divided between floors

[Joseph W. Doherty]

BTW, @pieterh's is nice approach, making the existing allocations more "visible" BUT a portion of it is NOT to scale.  It also has a "typo".

The yellow area is a /25 block (128 IPs), but a /28 (16 IPs) and the two /30s (8 IPs, 2 /30s combined), all 3 subnets, together, only use about 15% of the /25.  I.e. the two "free" blocks, are, possibly, misleading, if you think you're limited to just two address block allocations and/or only about 2/5, 40%, rather than about 85%, of the /25 is free.

The red area, typo, should have been 10.0.1.128/25.

[pieterh]

hi @Joseph W. Doherty , thanks for correcting the typo.
my image is delibarately not actual scale, this will run off the page
but i think it will serve it's purpose to display where how the total space is filled

[Joseph W. Doherty]

"my image is delibarately not actual scale, this will run off the page . . ."

Well, since the /24 and /25 blocks seem to be to scale, and since for those trying to understand address block allocating, I suggest you try to make this obvious, especially in this case as OP wishes to do further allocations.

A couple of specific suggestions, a note that the /25 lower half of the floor's /24, the /28, the two /30s and the two "free" blocks (BTW, just curious, why two free blocks?) are not to scale.  Or, perhaps, have an additional separate similar diagram showing that /25 allocation.  Or, perhaps show those subnet allocations to scale but with the text, external to the diagram.

Perhaps, my writing "nice approach" doesn't convey the approach is actually excellent!  I applaud you for using it.

However, because, I thought the actual presentation might be better, I thought it, overall, didn't rise to excellent, but was still nice.

Sorry for the critique, but as you've explained what you had in mind, I've just explained what I thought.

Lastly, for "delibarately" was that supposed to be "deliberately"?

[Joseph W. Doherty]

I believe I now fully understand your original question and your "delimma".

Yes, you can continue to add new VLSM subnets after your last /30, but NOT doing it EXACTLY as you initially did it.

To understand how to continue your VLSM subnetting, you need to understand address space allocation.  Such understanding will also make clear why your initial VLSM subnetting was done as it was.

IP address blocks, excluding host (/32), are always allocated in a power of two segments.

For example a /30, we know, comprises 4 IP sequential addresses, but NOT just any four sequential 4 IP addresses!

Each starting IP, for a /30, must be a multiple of 4 (which also is the required power of 2, 2 to 2nd power).  E.g. 0, 4, 8, 12, . . .

A valid set of IPs, for a /30 block, could be 10.0.0.20..23 (10.0.0.20/30), but NOT 10.0.0.21..24 or 10.0.0.23..26, etc.

Likewise for other subnet sizes.  For example, the starting IP for a /25 must be a multiple of 128 (2 to the 7th power).

Do not pass go, do not collect $200, until you understand the foregoing.

Next, you need to understand the implications of subnetting an address range.

Say I've provided you 10.0.0.0/23 and you've determined you need a /25, and 4 /30s out of it.  But I also tell you, the 4 /30s must be 10.0.0.20/30, 10.0.0.140/30, 10.0.1.120/30 and 10.0.1.200/30.  That shouldn't be a problem, right?  So, what will be your /25?

Problem allocating the /25, why?

What if I say you may reassign one of the /30s?  Does that help?  If so, what did you do and why?

What about if you can assign all the subnets as you wish?  What if you use your large to small approach?  Is it the only way?

If can't work out to this point, let me know.  If you can, you'll be ready to understand why you did what you did initially and how you might continue.  Let me know, and we can discuss what was described in earlier replies regarding continuing your VLSM allocations.

[pieterh]

Hi @Joseph W. Doherty  lets's not put all details to get my post as you want it
but let's help the original poster with his problem, you agree ?

[Joseph W. Doherty]

Sorry, no, I don't agree about not commenting on someone else's replies because clarity and accuracy, I believe, helps the OP AND possibly other readers.

Personally, I welcome corrections or suggestions to my replies because I do want to do the best for others, as I believe you do too.

To be clear, in this case, you only had one true error, which you thanked me for correcting.

As to the presentation of that one /25 block, my OPINION was the suballocations within the /25 might be unclear to OP or SOME readers so I further detailed what was going on in that address block. This I don't regret noting, but was this absolutely needed?  No, as honestly, this additional detail's value is subjective.

In hindsight I do regret using the adjective "nice" as, again in hindsight, it might be taken as meaning to disparage your reply, it was not so intended, and if it was so taken, I apologize.

Now, let's turn this around.  You thanked me for correcting the typo, but then basically explained/defended why you did what you did and it was done to serve YOUR purpose.  I fully believe all true!  But, how does this benefit OP or other readers?  IMO, it doesn't, but again, if my reply made you feel you needed to defend your reply, my reply was poorly worded.

Edit - PS:

Laugh, might my replies, to your's, be more of the same?  I.e. were they too unnecessary?

Possibly, but 1) I wanted to insure you understand my intent was to not denigrate your replies, and 2) similar might happen again, without "clearing the air".

[Joseph W. Doherty]

I'm looking at your attached PT file.

So your question is?

If just, can you add additional subnets without changing existing networks?  From the information you provided, the answer is yes.

If you ask, how should/can that be done?

That depends, again, on how many new IPs you need, and where you need them.