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

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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Comments

jdancer

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.

hennrizzler

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?

jdancer

You can read this TE thread about the biggest network guideline. It's really there to make your work easier.

http://www.techexams.net/forums/ccna-ccent/77690-biggest-subnet-first-rule-very-critical.html

gambler103

Hi,

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 192.168.129.0/24. This subnet is currently split up using VLSM into 255.255.255.240 or /28 subnets (as below).

192.168.129.0/28 USED
192.168.129.16/28 USED
192.168.129.32/28 SPARE
192.168.129.48/28 USED
192.168.129.64/28 SPARE

We are running out of address space in the 192.168.129.16/28 subnet so my question is, Is it possible to incorparate the currently spare subnet 192.168.129.32/28 into 192.168.129.16/28 therby creating 192.168.129.16/27 (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.

192.168.129.16 255.255.255.224 giving usable addresses of 192.168.129.17-46 (mask relates to the ‘16’ in the last octet).

OR

192.168.129.16 255.255.255.224 giving usable addresses of 192.168.129.1-30 (mask ignores the ‘16’ and relates back to the zero subnet address 192.168.129.0).

Any help much appreciated!

Thanks