Cloud & Platform

CIDR Subnet Calculator

Subnet arithmetic for VPC planning, counting the five addresses each cloud reserves rather than the traditional two.

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How to use this tool

  1. Paste the blocks you are planning, one per line, as address/prefix.
  2. Set a split size if you want the block carved into equal subnets — 24 to cut a /16 into 256 of them.
  3. Select Calculate subnets.
  4. Plan against the cloud usable count, not the traditional one. The difference is three addresses per subnet and it matters most on the small ones.

What cidr subnet calculator does

Textbook subnetting says a block loses two addresses: the network and the broadcast. Every major cloud takes five, for the network address, the router, DNS, a reserved address and the broadcast. On a /16 that difference is noise. On a /28 it is the difference between sixteen addresses and eleven, which is the difference between a subnet that fits the workload and one that does not.

Paste any IPv4 block and this gives the network and broadcast addresses, the full range, the mask, the traditional usable count and the cloud usable count side by side. It also flags the provider limits people hit — AWS rejects a VPC larger than /16 and a subnet smaller than /28 — and tells you when the address you pasted is a host inside the block rather than the block itself. Set a split size to carve the block into equal subnets and see the first few boundaries.

Frequently asked questions

On top of the network and broadcast addresses, AWS, Azure and Google each reserve three more per subnet for the router, DNS and future use. So a /24 gives 251 usable addresses rather than 254. On a large block that is noise; on a /28 it is eleven usable addresses instead of fourteen, which is the difference between a subnet that fits and one that does not.

AWS accepts /16 down to /28 for a VPC and /28 is the smallest subnet. A /16 gives 65,536 addresses and costs nothing to reserve, so the usual advice is to take one from private space and carve it generously — the expensive mistake is a VPC too small to grow into, because expanding one afterwards means adding secondary ranges and living with a fragmented plan.

RFC 1918 defines three: 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16. Anything else is publicly routable space, and using it privately means your network cannot reach the real owner of those addresses. The tool says which category each block falls in, since a block picked without checking is a problem that only surfaces when someone needs to reach that part of the internet.

No — this is IPv4 only. IPv6 subnetting works differently enough that the same arithmetic would mislead: allocations are effectively always /64 per subnet, address exhaustion is not a planning concern, and the reserved-address arithmetic that makes this tool useful does not apply in the same way.