If you are in a greenfield scenario (networks and MX's haven't actually been deployed yet) you could accomplish this using the "Unique Subnetting" option in the template which will then generate unique address ranges for each network based on the parameters you provide. The other possible option would be to use the Dashboard API interface for Networks and Group Policy which I would not recommend you do if this is your first attempt at using Dashboard APIs unless its a greenfield as well.
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