Subnet Calculator takes an IPv4 or IPv6 address and a prefix length and returns the network address, broadcast address, usable host range, usable and total address counts, and the mask in CIDR, dotted-decimal and wildcard form. It runs in your browser. To start from a dotted mask, convert it with the CIDR Notation Converter first.

How to read the result

The result table lists each derived value; for IPv4, usable hosts is the address range minus the network and broadcast addresses, so it is two less than the total for every prefix up to /30. A /31 is a point-to-point link with both addresses usable (RFC 3021) and a /32 is a single host.

Defender tips

Plan address space and VLAN boundaries before deployment rather than after; renumbering a live network to fix an undersized subnet is disruptive and error-prone.
Document every subnet allocation somewhere durable (not just in device configs); the Subnet Calculator and CIDR Notation Converter make the arithmetic easy, but only a record prevents two teams from claiming the same range.
Avoid overlapping ranges between sites connected by VPN or WAN; overlap is one of the most common causes of routing that works at each site individually but fails once they are connected.

FAQ

Does it support IPv6?

Yes. IPv6 has no broadcast address, so the result shows the first and last address, every address in the prefix counts as usable, and both the compressed and expanded forms of the address are given.

Why is the usable host count total minus 2?

For IPv4, the first address in a subnet is reserved as the network address and the last as the broadcast address, so neither can be assigned to a host, for any subnet larger than a /31.

What is VLSM and does this help with it?

Variable Length Subnet Masking is using different mask sizes within the same address space to avoid wasting addresses; this calculator is the arithmetic building block for planning a VLSM layout by hand.