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.

CIDR block

What is CIDR?

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.

Frequently asked

How do I read CIDR notation like /24?

The number after the slash is how many leading bits are the network portion. /24 means the first 24 bits are the network and the remaining 8 identify hosts.

How many hosts are in a /24?

A /24 holds 256 addresses, but two are reserved (the network and broadcast addresses), leaving 254 usable hosts. Paste any block above for exact counts, IPv4 or IPv6.

What are the network and broadcast addresses?

The network address is the first in the block (all host bits 0) and names the subnet; the broadcast address is the last (all host bits 1) and reaches every host. Neither is assignable to a device.