Subnet Calculator (IPv4 & IPv6) with CIDR Summarization

What it does

Subnet calculator: Enter an address with a prefix (192.168.10.37/26), with a netmask (10.0.0.1 255.255.240.0) or an IPv6 address (2001:db8::1/64). You get network address, netmask, wildcard mask, broadcast, first and last host, usable hosts, address type (private, public, CGNAT …), reverse DNS zone and binary view. “Split into subnets” divides the network into smaller subnets.

Summarize to CIDR: Paste any number of IP addresses, networks or ranges — for example attacker IPs from a firewall or web server log during a DDoS or brute-force attack. The tool removes duplicates, merges overlapping and adjacent ranges and returns the smallest possible list of CIDR blocks. Optionally round every IP up to a /24 to block whole networks.

Export formats: plain CIDR list (works as a Palo Alto External Dynamic List), FortiGate CLI with address objects and an address group, Palo Alto set commands, Cisco IOS ACL with wildcard masks, Cisco ASA object-group, Cisco prefix-list, ipset/iptables or nginx deny rules.

Range → CIDR: converts a range such as 203.0.113.17 – 203.0.113.200 into the exact matching CIDR blocks.

Find IP in list: shows which networks in a list contain a given IP — handy for finding which firewall object or customer rule matches an address.

CIDR, netmask and wildcard in a nutshell

The CIDR notation /24 states how many bits belong to the network part. /24 equals the netmask 255.255.255.0 and contains 256 addresses, 254 of them usable hosts. The wildcard mask is the inverted netmask (0.0.0.255), used mainly in Cisco ACLs and OSPF. A /31 is a special case for point-to-point links (RFC 3021) with two usable addresses; a /32 is a single host.

FAQ

Is my input stored? No. Everything runs in your browser; nothing is sent to a server.

Does it support IPv6? Yes, in every function — including summarization, range conversion and export.