CIDR / Subnet Calculator
Runs entirely in your browser — nothing you type is sent anywhere.
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Exact, even for IPv6
An IPv6 /0 contains 340282366920938463463374607431768211456 addresses. Most calculators show you 3.4e38 — a rounded float, not an answer. This one computes with exact integers throughout, so every count is the real number, however large.
Three details other calculators get wrong
If you type 10.0.0.5/24 we tell you that address is a host inside the block, not the network — the typo that quietly breaks firewall rules. A /31 has two usable addresses and a /32 has one (RFC 3021), not zero and minus-one. And IPv6 has no broadcast address, so we don't invent one. Leading zeros like 010.0.0.1 are refused rather than guessed, because different systems read them as different addresses. As with every tool here, the page is static and keeps working offline, verified by our automated tests on every release.