CIDR / Subnet Calculator

The number after the slash is the prefix length — how many leading bits of the 32-bit IPv4 address are fixed as the network portion. /24 fixes the first 24 bits (the first three octets), leaving 8 bits (256 addresses) for hosts, which is why 192.168.1.0/24 spans 192.168.1.0 through 192.168.1.255.

Usable hosts = 2^(32 − prefix) − 2 for any prefix from /0 to /30, because the first address in the range is reserved as the network address and the last is reserved as the broadcast address. /31 is a documented exception (RFC 3021): both addresses are usable host addresses on a point-to-point link. /32 is a single host with no usable range.

A wildcard mask is the bitwise complement (inverse) of the subnet mask — e.g. a /24's subnet mask 255.255.255.0 has wildcard mask 0.0.0.255. Subnet masks mark network vs. host bits for routing; wildcard masks are used the same way but are common in Cisco ACL and OSPF configuration syntax.

The broadcast address is the very last address in the block (all host bits set to 1) and is reserved for sending to every host on that subnet — it cannot be assigned to a device. The last usable address is one below it, the highest address you can actually assign to a host.