CIDR / Subnet Calculator

CIDR (Classless Inter-Domain Routing) notation writes an IP network as an address followed by a slash and a prefix length - for example 10.0.0.0/24. The prefix is the number of leading bits that make up the network portion of the address; the remaining bits identify hosts within that network.

A /24 means the first 24 bits are the network, leaving 8 bits for hosts - which is 256 addresses, or 254 usable ones. The smaller the prefix number, the bigger the network.

A subnet mask is the prefix expressed as a dotted-decimal number. A /24 prefix is the mask 255.255.255.0 - in binary, 24 ones followed by 8 zeros. The ones mark the network bits and the zeros mark the host bits.

The device finds the network address by performing a bitwise AND of the IP and the mask. The wildcard mask is the inverse (0.0.0.255 for a /24) and is used in router access-control lists and OSPF, where the bits to ignore are the ones set.

Every subnet reserves two addresses: the network address (all host bits 0, the first address) identifies the subnet itself, and the broadcast address (all host bits 1, the last address) reaches every host at once. Everything in between is assignable to devices, which is why usable hosts equals total addresses minus two.

Two exceptions: a /31 has no broadcast and gives 2 usable addresses for point-to-point links (RFC 3021), and a /32 is a single host - useful for firewall rules and routes.

Some ranges are reserved for private networks (RFC 1918) and are never routed on the public internet: 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16. The calculator labels each address as private, public, loopback, link-local, or another special-use type so you know what you are looking at.

1. Type an IPv4 address with a prefix, like 192.168.1.0/24, or just an address.
2. Drag the prefix slider to resize the network and watch every value update live.
3. Read off the network, broadcast, usable range, host count, mask, and wildcard.
4. Copy any single value, or copy the whole summary at once.