J01
Subnet calculator
CIDR arithmetic for IPv4 and IPv6 — network, broadcast, usable range, mask and subnetting.
WHAT IT WILL NOT DO
- Leading zeros are refused, in the address and in the prefix alike.
- Splitting lists at most 128 subnets, ranges at most 64 blocks; totals are still exact.
- Arithmetic only — no whois, DNS or reachability; the reserved-range list is a 2024 IANA snapshot.