VLSM - Variable Length Subnet Mask - best practice or requirement?

hennrizzlerhennrizzler Member Posts: 23 ■□□□□□□□□□
I've been learning about VLSM. While I knew subnetting and subnet masks, I've only come across VLSM and it makes sense to carve out subnets giving varying subnet masks to make more efficient use of the address block. My question is:

When using VLSM do you always assigned the biggest network first? I.E

150 devices
70 devices
20 devices

or does it not matter how I carve up the subnet? I hope I make sense.


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    jdancerjdancer Member Posts: 482 ■■■■□□□□□□
    I think the suggestion for biggest network first is to avoid a bunch of little fragmented networks later. Like two /30 networks separated by a /28 network.
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    hennrizzlerhennrizzler Member Posts: 23 ■□□□□□□□□□
    That makes sense. So it's only a suggestion/guideline then? I assume in production, you'll eventually get a requirement to build a new subnet with a larger network and this will cause the fragmentation you suggest?
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    jdancerjdancer Member Posts: 482 ■■■■□□□□□□
    You can read this TE thread about the biggest network guideline. It's really there to make your work easier.

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    gambler103gambler103 Registered Users Posts: 1 ■□□□□□□□□□

    Im still confused here and cant find a legitimate reason why you can use diffrent VLSM masks within a subnet.

    For example, we have an existing DMZ subnet of This subnet is currently split up using VLSM into or /28 subnets (as below). USED USED SPARE USED SPARE

    We are running out of address space in the subnet so my question is, Is it possible to incorparate the currently spare subnet into therby creating (and 15 more usable host addresses)??

    This obviously goes against the largest subnet first guidance but I cant find anything on the net that categorically tells me or explains to me why we cant use this subnetting?

    I think this question boils down to the fact of whether the mask used relates to the ip address being used or the subnet as a whole.

    e.g. giving usable addresses of (mask relates to the ‘16’ in the last octet).

    OR giving usable addresses of (mask ignores the ‘16’ and relates back to the zero subnet address

    Any help much appreciated!

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