IPv6
IPv6 is the current version of the Internet Protocol, with 128-bit addresses written in hexadecimal, such as 2001:db8::1.
What IPv6 is
IPv6 is the successor to IPv4. Its addresses are 128 bits long and are written as eight groups of four hexadecimal digits separated by colons, for example 2001:0db8:0000:0000:0000:0000:0000:0001. Two rules shorten that: leading zeros in a group can be dropped, and one run of all-zero groups can be replaced by ::. The same address becomes 2001:db8::1.
The address space is so large that providers hand out whole networks instead of single addresses. A home connection typically receives a /64 or larger, which means every device gets its own globally reachable address and NAT is no longer required.
Address types you will see
2000::/3is global unicast, the public internet.fe80::/10is link-local, present on every interface and never routed.fc00::/7is unique local, the IPv6 equivalent of private IPv4 ranges.::1is loopback.::ffff:203.0.113.7is an IPv4-mapped address, used when software handles both protocols through one socket.
Most devices also rotate a temporary "privacy" address every day or so, which is why your IPv6 address can change while your IPv4 address stays the same.
Why it matters for proxy and VPN users
Many proxies and some VPN configurations carry IPv4 traffic only. If your device also has working IPv6, a website that publishes an AAAA record can be reached over your real IPv6 connection while the IPv4 side goes through the tunnel. That is an IPv6 leak, and it exposes your true provider and country regardless of how good the IPv4 proxy is. Our anonymity score deducts 15 points when an IPv6 address is reachable and geolocates to a different country than the IPv4 address.
How to check
The IPv6 test contacts an IPv6-only host from your browser and reports whether a connection succeeded and from which address. The IP leak test combines that with the WebRTC and DNS checks. If you see an IPv6 address and did not expect one, disable IPv6 on the interface or choose a tunnel that carries both protocols.
A common misconception
Having no IPv6 is not a problem for privacy. "No IPv6 connectivity" in our results means nothing can leak over IPv6; it is the safest outcome for a proxied setup.