> ## Documentation Index
> Fetch the complete documentation index at: https://express-rate-limit.mintlify.site/llms.txt
> Use this file to discover all available pages before exploring further.

# Helpers

## `ipKeyGenerator(ip, ipv6Subnet=56)`

**Params:**

* `ip` (string) An IPv4 or IPv6 IP address
* `ipv6Subnet` (number|false) Subnet mask to apply to IPv6 addresses. Valid
  range is 1-120, recommend range is 48-64.

**Returns:** (string) IPv4 address unchanged, or IPv6 network address in CIDR
notation. (Unless second parameter is `false`, then IPv6 is also returned
unchanged).

This method is used by the default
[`keyGenerator`](./configuration#keygenerator) to apply the configured
[`ipv6Subnet`](./configuration#ipv6subnet), in order to properly rate-limit IPv6
users.

When using a custom [`keyGenerator`](./configuration#keygenerator) that uses the
user's IP for rate-limiting (even as a fallback), return the result of this
method rather than the IP address itself.

```ts theme={null}
import { rateLimit, ipKeyGenerator } from 'express-rate-limit
const limiter = rateLimit({
	// ...
	keyGenerator: (req, res) => {
        // use userID for authenticated users
        if (userIsAuthenticate(req)) {
            return req.userId
        }
        // fall back to IP address for unauthenticated users
        // use ipKeyGenerator to apply a /56 subnet to IPv6 users (IPv4 returned unchanged)
        return ipKeyGenerator(req.ip, 56)
    },
})
```