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TOPIC: VLSM question

VLSM question 7 years 5 months ago #29926

  • baretas
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Hi! I'm currently studying subnetting and VLSM subnet calculation and after seeing some examples and explanations on the VLSM subject I still couldn't answer this doubt:

Is there an "official" way to order the subnets in the VLSM?

I guess this is a stupid question, but in some places (cisco included) it's recommended that we start with the big blocks (/26 and so on). In other examples I saw putting the big subnets in the middle, others don't even start from the "0"...
I feel that this is a subjective choice and is open to free judgment. But my second doubt is if this is a subjective choice, then there are no right answers when assigning IPs to those subnets.
I hope that I explained this correctly... :P and thanks in advance for any answer.
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Re: VLSM question 7 years 5 months ago #29943

  • SteveP
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I'm not aware of any official guidelines but I've always started with the large subnets first (ip subnet zero is the default now) then worked my way through to the smallest subnets (/30 for P2P serial links).
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Re: VLSM question 7 years 5 months ago #29944

  • S0lo
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I guess the idea behind placing large subnets first is to try to avoid too much fragmentation in the address space after too many changes. And to ease changes done to small subnets.

Large subnets are usually less pron to changes in the future. For example, when you allocate a /24 subnet, this will hold 254 hosts, a relatively large number of hosts for today's known LANs. And it will usually have allot of unused IPs before it's totally consumed. Such subnet is usually sufficient for a long time and is less expected to be expanded or shrinked. On the other hand smaller subnets (say /28 for 14 hosts) hold less number of hosts (say for small departments). They are usually more dynamic and will probably need expanding/moving in the future.

Now, it's obvious that expanding/moving subnets at end of your address space is much easier than doing it at the start of your address space, since doing the later will probably require moving/changing all the following subnets at the end of the address space. This could also cause unused ranges (fragments) in your address space (after so many changes) which is a waste.
Studying CCNP...

Ammar Muqaddas
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Re: VLSM question 7 years 5 months ago #29945

  • baretas
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Thanks for all the answers! :)
I've been doing the exercises with the big blocks first (i thought it was the logical way to do), but now I'm more reassured.

Let's see if in the near future I can be equally helpful to other members. =D
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