CIDR Host Count Calculator

About this calculator

CIDR notation writes an IPv4 subnet as an address followed by a slash and a prefix length (e.g. /24) that tells you how many of the 32 address bits are fixed for the network, leaving the rest for hosts. The block holds 2^(32−prefix) total addresses, but two of those — the network address and the broadcast address — can’t be assigned to a device, so usable hosts equal 2^(32−prefix) − 2 (a /31 or /32 is a special case with no such reservation).

The same prefix length also corresponds to a dotted-decimal subnet mask (a /24 is 255.255.255.0, a /27 is 255.255.255.224, and so on), which routers and operating systems actually use to decide which addresses share a network. Network engineers and system administrators use this conversion constantly when carving a larger network into subnets, sizing a subnet for a given number of devices, or documenting a VLAN’s addressing plan.