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Check whether an IPv4 or IPv6 address belongs to VPN infrastructure, no key needed.
VPNDetection API is a free Security API that requires no API key, is served over HTTPS and supports CORS, so it can be called from the browser.
The VPNDetection HTTP API tells you what we know about an IP address: one request, one address, and whether it belongs to VPN infrastructure.
Details from docs.vpndetection.io.

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The VPNDetection API is provided by VPNDetection Documentation (docs.vpndetection.io). The VPNDetection HTTP API tells you what we know about an IP address: one request, one address, and whether it belongs to VPN infrastructure.
Yes. The VPNDetection API is listed in the Security category of the public-apis directory, which only includes APIs that are free or have a free tier. Check its documentation for rate limits and paid plans.
No. The VPNDetection API requires no authentication, so you can start making requests without signing up.
Yes. The VPNDetection API supports CORS, so front-end JavaScript can call it directly.
Yes. The VPNDetection API is served over HTTPS, so it's safe to use in production.