Divide a VPC CIDR across availability zones and subnet tiers and get the real usable address count — not the arithmetic one. Everything runs in your browser; nothing is transmitted or stored.
What this accounts for that mental arithmetic does not
AWS reserves five addresses in every subnet: the network address, the VPC router, the DNS resolver, one held for future use, and the broadcast address. A /24 therefore gives 251 usable addresses rather than 256, and a /28 gives 11 rather than 16. The absolute loss is constant, which means it matters most in the small subnets where people are least likely to allow for it.
The planner also allocates tier-major — every subnet of a tier is contiguous across the zones. That is deliberate: contiguous per-tier ranges summarise to a single prefix in route tables and security group rules, where interleaved allocations do not. It is why the planner will skip address space to keep a larger tier on its natural boundary rather than packing blocks tightly.
What it deliberately does not do
- It does not reserve space for future growth on your behalf. The free space it reports is the space you still have to allocate deliberately.
- It does not know about your on-premises ranges, peered VPCs or future acquisitions. Overlapping address space is the most expensive VPC mistake, and only you can see the whole estate.
- It does not model IPv6, which is allocated on a different basis entirely.
- It is a planning aid, not a provisioning tool. Verify sizes against current AWS documentation before building.
The design reasoning behind these choices is covered in VPC and subnet design.