CIDR Calculator

About this CIDR calculator

CIDR notation writes an address and a prefix length as 192.168.1.10/24. The number after the slash says how many of the leading bits are the network portion; the remaining bits identify the host within that network. An IPv4 address has 32 bits, so /24 leaves 8 host bits and describes a block of 2⁸ = 256 addresses, of which the first is the network address and the last is the broadcast address — leaving 254 assignable to hosts.

Two prefixes break the usual subtraction. A /31 is defined by RFC 3021 for point-to-point links and has 2 usable addresses rather than 0, and a /32 names a single host. Everything from /30 downward follows the familiar "total minus two" rule. The reference table below gives the mask, the wildcard (the inverse mask used by Cisco-style ACLs), the block size and the usable host count for every prefix from /8 to /32.

Private address space is defined by RFC 1918: 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16. This tool also flags the other non-routable blocks — loopback 127.0.0.0/8, link-local 169.254.0.0/16, carrier-grade NAT 100.64.0.0/10 and multicast 224.0.0.0/4 — because "is this address routable on the internet" is usually the question you actually have. IPv6 blocks are supported too; note that IPv6 has no broadcast address, so no two addresses are subtracted there.