IPv6 attribution: /64s, temporary addresses and why the answer is fuzzier
IPv6 changes what "an IP" means: users control most of the address, clients rotate it, and a /64 is a single household. Here is what that does to attribution.
One household, not one address
In IPv4, a residential line usually holds one public address, sometimes a small block. In IPv6, an ISP typically delegates a /64 to one household — 18 quintillion addresses. The customer controls everything inside it.
That breaks two assumptions built into IPv4 tooling:
- "one IP = one connection" is false; a single customer can use thousands of addresses.
- "blocking the address blocks the user" is false; they can rotate inside their /64 at will.
Privacy addresses rotate by design
RFC 4941 (now updated by RFC 8981) defines temporary addresses: clients generate a random interface identifier, use it for hours or days, then switch. Two sessions from the same laptop can share a /64 but never the same address.
Consequences for an IP check:
- A /64 is the stable unit, not the address. If you must group traffic, group by /64.
- Seeing the same IPv6 twice is not evidence of the same user; seeing the same /64 is stronger.
- Your history feature is less useful on IPv6 unless it keys on the prefix.
What is still usable
- Prefix ownership still maps to the ISP, so ASN and network type remain meaningful.
- The /64 prefix can identify a household for a session window.
- No NAT means the client can be reached directly, which some systems treat as a different risk profile than CGNAT IPv4.
Why our checker is IPv4-only right now
The databases we ship are IPv4. IPv6 lookup needs an address parser, a different prefix-length source and a separate test set, and the privacy semantics above mean the verdict must be phrased differently. Rather than return a half-right answer, the API returns a clear "not enabled in this build" note for IPv6.
If you need IPv6 in production today, treat it as a separate datapoint: check the /64, then the address, and never assume the two behave like IPv4.