Subnetting of a /24 network due to using VLANs

Question

We're in the process of dividing a /24 network into a /25 network, a /26 network and a /27 network.

Our original plan was as follows:

10.97.21.0/24 (original network) to

10.97.21.0-31 /27

10.97.21.32-159 /25

10.97.21.160-223 /26

10.07.21.224-255 /27

However, I have been told that this won't work because it's imperative to always start with the largest network first. Why is that the case?

You don't have to start with the largest, but it will help you to not make the mistake you made. Your /25 is not on a bit boundary, as @datagramnetwork explains in his answer. — Ron Trunk, Feb 06 '17 at 12:52

Datagram.Network · Accepted Answer · 2017-02-06T11:02:28.543

6

Is not gonna work as: 10.97.21.32/25 is part of 10.97.21.0/25 network which is overlapping with your 10.97.21.0/27

10.97.21.160/26 is part of 10.97.21.128/26 which is overlapping again with other of yours network.

That should work:

10.97.21.0/25

10.97.21.128/26

10.97.21.192/27

10.97.21.224/27

edited Feb 06 '17 at 11:02

answered Feb 06 '17 at 10:53

Datagram.Network

score 3 · Answer 2 · edited Apr 13 '17 at 12:57

It is not imperative to always start with the largest network but it is easier if you're not accustomed with sub-netting.

You can take it this way:

Divide your 10.97.21.0/24 network into 2 /25 networks:

10.97.21.0/25
10.97.21.128/25

Since you need a /25, keep one and further subnet the other one, once again in two networks:

10.97.21.0/25
10.97.21.128/26
10.97.21.192/26

You need a /26, so you keep the first one and divide the other one in two:

10.97.21.0/25
10.97.21.128/26
10.97.21.192/27
10.87.21.224/27

as you can see we get the solution given by @Datagram.Network, but if choose instead to divide the first subnet and keep the second we can also have

10.97.21.0/27
10.97.21.32/27
10.97.21.64/26
10.87.21.128/25

2 Answers2