CIDR / Subnet Calculator

Enter an IPv4 or IPv6 address with a prefix length (10.0.0.5/20), a dotted subnet mask (10.0.0.5 255.255.240.0) or on its own. The calculator shows the network, subnet and wildcard masks, broadcast address, first and last address, usable host range, total and usable counts, the address type from the IANA special-purpose registries and the reverse DNS zone. All math uses exact big-integer arithmetic, so IPv6 /0 and IPv4 /0 are as precise as a /24.

IPv4 /0 to /30 excludes the network and broadcast addresses. A /31 has two usable addresses for point-to-point links (RFC 3021) and no broadcast; a /32 is a single host route. IPv6 has no broadcast, so every address in the prefix is counted, and /127 is noted as a point-to-point link (RFC 6164).

Choose a longer prefix under Split into to list the subnets, for example 10.0.0.0/16 into sixteen /20 networks. Splits are limited to 20 extra bits (1,048,576 subnets); the list renders the first 4,096 rows and always shows the real total, and Copy subnets or Download CSV include up to 65,536 subnets.

IPv4 must be four decimal octets: leading zeros, short forms such as 10.1 and hex are rejected because systems read them differently. IPv6 follows RFC 4291 text forms including :: and an embedded IPv4 tail; results use the RFC 5952 canonical form. Zone ids (%eth0) are not accepted. This is math only: nothing is pinged, resolved or checked against a registry of assignments, and addresses you type are not saved.