CIDR Planning
Allocating non-overlapping address ranges across an estate in advance, because ranges cannot be resized and overlaps prevent connectivity.
The most expensive networking decision to get wrong, and it is made on day one by whoever creates the first VPC.
Two failure modes. Too small: a /24 VPC runs out when a Kubernetes CNI assigns an address per pod,
and the range cannot be expanded in place — the remedy is a new VPC and a migration. Overlapping:
two networks with the same range cannot be peered or connected by VPN, so an acquisition, a partner
integration or a merge of two teams' environments becomes impossible without renumbering.
The practice: allocate from a central register, reserve generously (private address space is free), give each environment and region a distinct block, and leave room for growth between allocations. Record the allocations somewhere authoritative before anyone provisions.
Check against the corporate network and any likely acquisition target's ranges, not only your own.