CIDR Calculator
What is CIDR Calculator?
CIDR Calculator is a free online subnet calculator for IPv4 and IPv6. Paste CIDR notation like 10.0.0.0/8 or 2001:db8::/32 and get the network address, broadcast, first and last usable hosts, subnet and wildcard masks, and exact address counts — computed with arbitrary-precision integers, so IPv6 totals like 2^64 are exact instead of floating-point guesses. Switch to IP range mode to decompose any start–end range into the minimal set of CIDR blocks, the exact operation firewall rules and allowlists need.
Features
- Full IPv4 detail — network, broadcast, first/last usable, total and usable hosts, dotted subnet mask and wildcard mask, with correct RFC 3021 /31 (two usable, no broadcast) and /32 handling
- Real IPv6 support — RFC 5952 canonical output, embedded IPv4 tails like
::ffff:192.0.2.1, and exact 128-bit host counts; /127 follows RFC 6164 - IP range to CIDR — summarizes any range into the smallest exact set of non-overlapping CIDR blocks, instantly, even for ranges the size of
::/0 - Host-bits detection —
10.0.0.1/8is flagged with a one-click fix to10.0.0.0/8instead of silently misleading output - Strict parsing — leading-zero octets, short forms, zone IDs and bracketed addresses are rejected with clear reasons, not misread
How to use
Pick CIDR notation mode and paste a block such as 192.168.0.0/24 — every field appears with its own copy button. If the address has host bits set, click the suggested network address to normalize it. Pick IP range mode and enter a start and end address to get the minimal CIDR list plus the total address count.
Privacy
All math runs locally in the browser UI. Addresses you analyze on the page are not uploaded or logged.