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: (original network) to /27 /25 /26 /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?

  • 2
    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 6 '17 at 12:52

Is not gonna work as: is part of network which is overlapping with your is part of which is overlapping again with other of yours network.

That should work:

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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 network into 2 /25 networks:

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

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

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

For more information you may find this answer to How do you calculate the prefix, network, subnet, and host numbers? interesting

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