IP and CIDR Subnet Calculator

Work out network, broadcast, ranges, masks and host counts for IPv4 and IPv6, split a block into subnets, or summarise a list of addresses into the fewest CIDRs.

Runs locally

Everything happens in your browser. What you paste or drop here is never uploaded, logged or stored.

IPv4 or IPv6 address with prefix

Every number a subnet has

Enter an address with a prefix. IPv4 also accepts a dotted netmask such as /255.255.255.0. The calculator shows:

the network and last address;
the broadcast address, for IPv4;
the usable host range and counts;
the mask and wildcard (the inverse used in ACLs);
the binary layout;
the reverse-DNS zone.

If you typed a host address rather than the network, it says so. Edge cases follow the standards:

a /31 has two usable addresses on point-to-point links (RFC 3021);
a /32 is a single host;
IPv6 has no broadcast, and addresses are written in the canonical compressed form of RFC 5952.

What kind of address is it?

Each block is placed in the special-purpose registry: RFC 1918 private space, carrier-grade NAT (100.64.0.0/10), loopback, link-local, the documentation ranges, multicast, IPv6 unique-local, NAT64 and IPv4-mapped. That matters in firewall rules, where "private" and "public" are often confused. The table follows the IANA registries.

Splitting and summarising

Split a block into equal smaller subnets, for example a /22 into four /24s. Summarise goes the other way. It takes a messy list of addresses, CIDRs and "first - last" ranges, merges what overlaps or touches, and returns the smallest set of CIDR blocks covering exactly the same addresses. That is useful for compacting allow-lists.

Frequently asked questions

Why are leading zeros rejected?

Because 010.0.0.1 means 10.0.0.1 to some parsers and 8.0.0.1 (octal) to others. Rejecting it is safer than guessing.

Does this look up the address?

No. Everything is arithmetic in your browser; no address is sent anywhere.

How large can the numbers get?

Exact at any size - an IPv6 /0 has 2^128 addresses, and all counts use arbitrary-precision integers.

References

Related tools

What this tool can and cannot tell you. It reports what is present in the input you provide. It cannot see anything you did not give it, and a clean result means nothing was found in that input - not that the wider system is secure.

Security guidance here follows current published sources - OWASP, MDN, the relevant RFCs, NIST, CISA, FIRST and MITRE - which are linked beside the specific claims they support.