Subnet / CIDR Calculator
Paste a CIDR block — IPv4 or IPv6 — and get the network, broadcast, mask, host range, and counts. All computed in your browser.
A /n suffix that says how many leading bits are the network — the rest are hosts.
CIDR (Classless Inter-Domain Routing) writes a network as an address plus a /n prefix length — the number of leading bits that identify the network. /24 means the first 24 bits are the network and the last 8 are host space (256 addresses; 254 usable, since the first is the network address and the last is broadcast). /31 is the special point-to-point case (2 usable hosts, no broadcast — RFC 3021), and /32 is a single host.
IPv6 has no broadcast address and so much space that "usable hosts" is effectively the whole block (this tool reports the count as a power of two). You can also enter a dotted mask instead of a prefix — 10.0.0.0/255.255.255.0 resolves to /24. Everything is computed locally; nothing is sent anywhere.