IPv4 Subnet Calculator

Works on prefixes (10.0.0.0/22) and on an address with a dotted mask (10.0.0.0 255.255.252.0). Results update as you type, and the URL always carries the current input so you can paste it into a ticket or a change request.

What the fields mean

Network and broadcast are the first and last addresses of the block. Neither is assignable to an interface in conventional subnetting, which is where the “minus two” in usable host counts comes from.

Wildcard mask is the bitwise inverse of the subnet mask. Cisco ACLs and OSPF network statements take a wildcard rather than a netmask, so 0.0.0.255 there means the same range as 255.255.255.0 elsewhere.

Usable hosts follows the two exceptions worth remembering. A /31 carries two addresses and reserves neither, per RFC 3021 — that is the normal choice for point-to-point links. A /32 is a single host route, typically a loopback or an anycast service address.

Splitting a block

Pick a target prefix length in Split into and the calculator lists every resulting subnet with its usable range and broadcast address. The list is capped at 256 subnets, which covers the splits people actually deploy from a single view.

If you need subnets of different sizes rather than equal ones, allocate largest first and keep each one aligned to its own size. That is variable length subnet masking, and doing it in size order is what keeps the result summarisable into a single announcement later.

Before you deploy the result

A calculation tells you the block is mathematically valid. It does not tell you whether the block is usable in production:

• Check the space is not reserved or bogon — filters at the edge silently drop those announcements.
• Confirm the registry record matches the intended holder with WHOIS and RDAP.
• Record the allocation in your address plan rather than only in the ticket.

Looking up rather than computing? The CIDR chart has every prefix length in one table. Need address space rather than a calculation? See IPv4 services.