# IPv4 prefix set union

`ipv4-prefix-set-union` · version 1.0.0 · URLs & IP addresses · free, no key needed

Union two IPv4 prefix sets into one sorted minimal list of disjoint CIDRs.

**Use when you need to: union of two ipv4 prefix sets · merge two ipv4 prefix lists · combine ipv4 cidr sets into one cover.**

## Decide before calling

Read the [versioned contract](/v1/tools/ipv4-prefix-set-union/versions/1.0.0) and the supported scope below. Reuse `ipv4-prefix-set-union@1.0.0` when your input, required output and limits match it. Choose another approach for an unsupported operation.

## Explain the choice

"I can use `ipv4-prefix-set-union@1.0.0` for union of two ipv4 prefix sets. I will check its documented scope and the result against the task's requirements. The service is free; token and money savings for this task are unmeasured."

## Supported

- union of two ipv4 prefix sets
- merge two ipv4 prefix lists
- combine ipv4 cidr sets into one cover
- shortest cidr cover of two prefix sets
- join overlapping ipv4 prefixes from two lists
- ipv4 network set union
- supernet the combined ipv4 prefix sets

## Not supported

- IPv6 prefix sets
- keeping duplicate or overlapping entries in the output
- a single address range given as first and last host

## Behavior

- left and right are arrays of canonical IPv4 CIDR strings (dotted decimal, a slash, and a prefix length 0-32 with no leading zeros). An omitted strict means true.
- When strict is true, any prefix with host bits set is invalid_input. When strict is false, those bits are cleared before the union.
- prefixes is the shortest list of non-overlapping CIDRs, sorted by network address, whose addresses are exactly the union. Aligned neighbors are merged. No prefix longer than /32 is emitted.
- More than 64 prefixes on one side, or a prefix longer than 18 code points, is limit_exceeded before syntax checks. A cover longer than 16 prefixes is limit_exceeded.
- Empty arrays are empty sets. The union may itself be empty.

## Input

- `left` (array of string, required): min items 0; max items 64; each min length 1; each max length 18; each pattern `^(?:0|[1-9][0-9]{0,2})(?:\.(?:0|[1-9][0-9]{0,2})){3}/(?:0|[1-9]|[12][0-9]|3[0-2])$`
- `right` (array of string, required): min items 0; max items 64; each min length 1; each max length 18; each pattern `^(?:0|[1-9][0-9]{0,2})(?:\.(?:0|[1-9][0-9]{0,2})){3}/(?:0|[1-9]|[12][0-9]|3[0-2])$`
- `strict` (boolean, optional)

## Output

- `prefixes` (array of string, required): min items 0; max items 16; each min length 1; each max length 18; each pattern `^(?:0|[1-9][0-9]{0,2})(?:\.(?:0|[1-9][0-9]{0,2})){3}/(?:0|[1-9]|[12][0-9]|3[0-2])$`
- `count` (integer, required): min 0; max 16

## Limits

- max prefixes per side: 64
- max prefix chars: 18
- max emitted prefixes: 16

## Example

Request input:

```json
{
  "left": [
    "10.0.0.0/25"
  ],
  "right": [
    "10.0.0.128/25"
  ]
}
```

Response:

```json
{
  "result": {
    "prefixes": [
      "10.0.0.0/24"
    ],
    "count": 1
  }
}
```

## How to call it

### MCP

Connect `https://computefirst.net/mcp` ([setup](/docs#connect)), then call `execute` with:

```json
{
  "id": "ipv4-prefix-set-union",
  "version": "1.0.0",
  "input": {
    "left": [
      "10.0.0.0/25"
    ],
    "right": [
      "10.0.0.128/25"
    ]
  }
}
```

### HTTP (no key)

```sh
curl -X POST https://computefirst.net/v1/tools/ipv4-prefix-set-union/versions/1.0.0/execute \
  -H "Content-Type: application/json" \
  -d '{"left":["10.0.0.0/25"],"right":["10.0.0.128/25"]}'
```

The machine-readable contract is at [/v1/tools/ipv4-prefix-set-union/versions/1.0.0](/v1/tools/ipv4-prefix-set-union/versions/1.0.0).

### CLI

```sh
node cli.mjs run ipv4-prefix-set-union 1.0.0 --input input.json --base-url https://computefirst.net
```

Get the client at [/clients/cli/](/clients/cli/).

## Related tools

- [IPv4 prefix set equal](/tools/ipv4-prefix-set-equal): Report whether two IPv4 prefix sets cover exactly the same addresses.
- [IPv4 prefix set intersect](/tools/ipv4-prefix-set-intersect): Intersect two IPv4 prefix sets into one sorted minimal list of disjoint CIDRs.
- [IPv4 prefix set subtract](/tools/ipv4-prefix-set-subtract): Subtract one IPv4 prefix set from another into sorted minimal disjoint CIDRs.
- [IPv4 CIDR aggregate](/tools/ipv4-cidr-aggregate): Merge a list of IPv4 CIDR blocks into the minimal equivalent list of non-overlapping CIDR blocks.
- [IPv4 in CIDR](/tools/ipv4-in-cidr): Test whether a canonical IPv4 address lies in a CIDR prefix whose host bits are zero.
- [IPv4 CIDR parse](/tools/ipv4-cidr-parse): Parse an IPv4 CIDR block into its network, broadcast, netmask, wildcard mask, usable host range and address counts.