Free IPv4 and IPv6 subnet calculator

How to use the IPv4 and IPv6 subnet calculator

1. Enter an IPv4 or IPv6 address and CIDR prefix, such as 192.168.1.10/24 or 2001:db8::42/64.
2. The calculator recognizes the address family automatically. IPv4 prefixes run from /0 through /32; IPv6 prefixes run from /0 through /128.
3. Select “Calculate subnet” to see the network prefix, mask, address range, address type, and adjacent equally sized networks.
4. Use “Copy results” when you need a plain-text summary for a ticket, configuration note, or change plan.

The calculator accepts unambiguous dotted-decimal IPv4 plus compressed, expanded, and IPv4-embedded IPv6 notation. IPv6 results are normalized to the concise lowercase form recommended by RFC 5952. Interface-specific zone IDs such as %eth0 are intentionally excluded because they do not change the numeric subnet.

Everything runs locally in the browser; no address, result, or network plan is sent to Bitfoo or added to the page URL.

What the subnet calculator results mean

CIDR notation combines an address with a prefix length. In 192.168.1.0/24, the first 24 of IPv4's 32 bits identify the network. In 2001:db8::/64, the first 64 of IPv6's 128 bits identify the network prefix. IPv4 classless prefix notation is defined in RFC 4632, while IPv6 address and prefix notation is defined in RFC 4291.

The mathematical address range does not prove that every address is assignable in a particular cloud, hosting, or network platform. Providers may reserve addresses for gateways, DNS, platform services, subnet-router anycast, or other infrastructure.

Common IPv4 CIDR prefixes and subnet masks

A larger prefix leaves fewer host bits and therefore creates a smaller subnet. The conventional usable-host count below subtracts the network and broadcast addresses through /30; /31 and /32 use their modern special-case behavior.

Common IPv6 prefixes and what they mean

IPv6 addresses contain 128 bits and use CIDR prefix lengths directly. There is no IPv6 broadcast address; multicast replaces broadcast behavior. The calculator therefore reports the complete prefix range instead of subtracting a network and broadcast address. These fundamentals come from the IPv6 Addressing Architecture in RFC 4291.

The “Address use” result recognizes common assignments in the current IANA IPv6 Special-Purpose Address Registry, including documentation, unique-local, link-local, loopback, multicast, translation, benchmarking, and transition ranges. A classification describes the address itself; it does not test live reachability or confirm a provider's routing policy.

Why IPv4 /31 and /32 subnets are different

A traditional host-count formula subtracts two addresses for the network and directed broadcast. Applying that rule to every prefix incorrectly reports that /31 and /32 have no usable addresses.

RFC 3021 defines /31 addressing for IPv4 point-to-point links. Both addresses are interpreted as host addresses on that link, with no separate network or broadcast reservation. A /32 identifies one exact IPv4 address and is commonly used for host routes, loopback interfaces, and precise firewall or routing entries.

This calculator reports two usable point-to-point endpoints for /31 and one address for /32. Whether a particular device or provider supports those uses is still determined by that platform.

Private and special-purpose IPv4 address ranges

The “Address use” result recognizes public unicast addresses plus common entries from the IANA IPv4 Special-Purpose Address Registry, including private, shared, loopback, link-local, documentation, benchmarking, multicast, and reserved ranges. The three private-use blocks are:

A classification describes the entered address, not whether it is routed or reachable in a particular network. Provider policy, firewalls, routes, address translation, and platform reservations still determine how an address can be used.

Common subnet calculator questions

Continue planning and operating a network