Build the calculator entirely in the browser: parse an IPv4 address and explicit CIDR prefix, derive the mask and network with exact 32-bit arithmetic, then convert the results back to dotted decimal. Local calculation can keep the address out of a calculation server request, but “zero-knowledge” is not a security guarantee by itself; the page’s scripts, analytics, hosting, and URL behavior also matter.
What the calculator needs to calculate
An IPv4 address is 32 bits, conventionally written as four decimal octets. CIDR writes the prefix length after a slash: for example, 192.168.1.130/24. The prefix says how many leading bits identify the network; the remaining bits are the host portion. RFC 4632 describes classless addressing and examples including 172.16.0.0/16 and 192.168.99.0/24. Do not infer a prefix from historical Class A, B, or C ranges; the prefix must be supplied explicitly.
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For prefix p, the mask is p one-bits followed by 32−p zero-bits. Bitwise AND of the address and mask gives the network address. Clearing the network bits gives the host portion. The valid prefix range is 0 through 32, inclusive: /0 has no network bits and /32 has no host bits.
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Parse and validate the input
Accept exactly four decimal octets separated by three dots, and a separate explicit prefix from 0 to 32. Reject malformed input rather than trying to repair or reinterpret it. This strict validation is an implementation choice; the four-octet address form comes from the IPv4 specification, RFC 791.
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- Reject missing or extra octets, non-decimal characters, and octet values outside 0–255.
- Reject a missing prefix or a prefix outside 0–32.
- Do not guess a prefix from the address’s first octet or legacy address class.
Implement the calculator
This function keeps the address, mask, and bitwise operations as BigInt values. Its output includes the dotted-decimal mask, network address, host portion, broadcast address, and the block size. JavaScript’s BigInt supports exact integer arithmetic beyond the precision range of Number; the two types have distinct rules and should not be mixed implicitly. See MDN’s BigInt documentation.
const MAX32 = (1n << 32n) - 1n;
function parseIPv4(text) {
const parts = text.split(".");
if (parts.length !== 4 || parts.some(part => !/^d+$/.test(part))) {
throw new Error("Enter four decimal octets, such as 192.168.1.130.");
}
const octets = parts.map(Number);
if (octets.some(octet => octet < 0 || octet > 255)) {
throw new Error("Each octet must be from 0 through 255.");
}
return octets.reduce((value, octet) => (value << 8n) | BigInt(octet), 0n);
}
function toIPv4(value) {
return [24n, 16n, 8n, 0n]
.map(shift => Number((value >> shift) & 255n))
.join(".");
}
function calculateSubnet(addressText, prefixText) {
if (!/^d+$/.test(prefixText)) {
throw new Error("Enter a CIDR prefix from 0 through 32.");
}
const prefix = Number(prefixText);
if (!Number.isInteger(prefix) || prefix < 0 || prefix > 32) {
throw new Error("Enter a CIDR prefix from 0 through 32.");
}
const address = parseIPv4(addressText);
const mask = prefix === 0 ? 0n : (MAX32 << BigInt(32 - prefix)) & MAX32;
const network = address & mask;
const host = address & (MAX32 ^ mask);
const blockSize = 1n << BigInt(32 - prefix);
const broadcast = network | (blockSize - 1n);
return {
prefix,
mask: toIPv4(mask),
network: toIPv4(network),
host: toIPv4(host),
blockSize: blockSize.toString(),
broadcast: toIPv4(broadcast)
};
}The mask expression shifts a 32-bit all-ones value left by the number of host bits and clips the result back to 32 bits. The explicit /0 branch avoids relying on a shift edge case; for /32, the shift is zero and the result is all ones. The block size is 232−prefix. The network is the address with host bits cleared, and the broadcast address is the last address in that block.
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Present ranges without overstating usability
Label the broadcast address as a calculated range endpoint, not as a promise that every address in a block is usable on a real network. Many familiar IPv4 subnet conventions treat the first address as the network identifier and the last as broadcast, with addresses between them usable for hosts. That convention is not universal across prefix sizes or operational contexts: in particular, /31 and /32 require care. RFC 4632 addresses CIDR and routing, not every local assignment policy. If the tool displays a “usable host range,” document the convention it uses and verify that convention for the intended environment rather than presenting it as universal.
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Verify boundary cases and examples
Use deterministic cases to check the arithmetic and validation behavior before presenting results to users. These expected values follow from the definitions; they are not a claim that this example has been independently tested.
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| Input | Expected mask | Expected network | What it checks |
|---|---|---|---|
192.168.1.130/24 |
255.255.255.0 |
192.168.1.0 |
Common octet-aligned prefix |
10.0.0.1/8 |
255.0.0.0 |
10.0.0.0 |
Network boundary at the first octet |
192.168.1.130/19 |
255.255.224.0 |
192.168.0.0 |
Prefix bits crossing an octet boundary |
203.0.113.7/0 |
0.0.0.0 |
0.0.0.0 |
No network bits; all address bits are host bits |
203.0.113.7/32 |
255.255.255.255 |
203.0.113.7 |
All address bits are prefix bits |
Also test invalid prefixes such as -1 and 33, malformed addresses, extra components, and octets above 255. If you add a usable-host output, test its policy separately for /31 and /32.
What “zero-knowledge” can honestly mean
A browser-only calculation means the arithmetic happens on the user’s device. It does not prove that the entered address is invisible to every other part of the page. Directly included third-party scripts can access other scripts and data on the page, and analytics, server requests, browser persistence, or query strings can expose inputs. MDN’s guidance on privacy on the web recommends minimizing collected data and managing third-party resources.
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- Keep the address and prefix in memory; do not put them in the URL or persist them unless necessary and clearly disclosed.
- Do not include the inputs in analytics events, error reports, or server requests.
- Review the deployed page’s scripts and network behavior, especially third-party resources, before making a privacy claim.
Only after checking the actual deployed page can an operator accurately say: “The subnet calculation runs in your browser; the address you enter is not sent to our server for calculation.” That describes a specific data flow, not a cryptographic zero-knowledge proof. RFC 1918 defines private IPv4 address allocation, but private-address status does not change how this calculator parses or computes a prefix.
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