IP Subnet Calculator (IPv4 & IPv6 CIDR)

How to use the subnet calculator

Enter an address and its mask. IPv4 takes a prefix (192.168.1.10/24), a netmask (192.168.1.10 255.255.255.0) or a Cisco wildcard mask (10.1.2.3 0.0.255.255). IPv6 takes a prefix, in any valid spelling: 2001:db8:abcd:12::1/64, the full eight groups, […] brackets, a zone such as %eth0 or an embedded IPv4 address like ::ffff:192.0.2.1. Results update as you type.

Below the calculator you can split the network into equal subnets (by count, hosts per subnet or new prefix, with a CSV export), turn an address range into CIDR blocks, and summarize a list of networks.

How the numbers are worked out

The prefix length says how many leading bits name the network; the rest number the addresses in it. The netmask is that many 1 bits, the network address is the address AND the mask, and the last address has every host bit set. An IPv4 block of prefix p holds 2^{32 − p} addresses, of which 2 fewer are usable hosts: the network and broadcast addresses. /31 point-to-point links (RFC 3021) and /32 host routes are the exceptions.

IPv6 has no broadcast address, so every address in a block counts. A /64 holds 2⁶⁴ addresses and is the standard size for one LAN: stateless autoconfiguration (SLAAC) needs it. Sites typically get a /48 or /56, which is 65,536 or 256 /64 networks. Use /127 for router-to-router links (RFC 6164).

Writing IPv6 addresses

RFC 5952 fixes one canonical text form: lowercase, no leading zeros in a group, and the longest run of two or more zero groups replaced by ::, so 2001:0db8:0000:0000:0000:ff00:0042:8329 becomes 2001:db8::ff00:42:8329. The expanded form writes all 32 hex digits, which helps when comparing addresses. The reverse DNS name lists those digits backwards under ip6.arpa.

IPv4 private ranges are 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16 (RFC 1918); 100.64.0.0/10 is carrier-grade NAT space. The page also recognizes loopback, link-local, multicast, documentation and reserved blocks of both families.

Ranges and summaries

Range to CIDR covers a first-to-last address range with the fewest aligned blocks, the way firewall and cloud rules need it: 192.168.1.10 to 192.168.1.20 is 192.168.1.10/31, 192.168.1.12/30, 192.168.1.16/30 and 192.168.1.20/32.

Summarize merges a list of networks and addresses (one per line) into the fewest CIDR blocks that cover exactly the same addresses, and shows the single smallest supernet containing them all, with how many extra addresses it adds. That is what route aggregation and IP allow-lists need.