Six checks run from your browser against our servers in Dhaka: whether you can reach a site that offers both IPv4 and IPv6, which one your browser picks, how quickly it gives up on broken IPv6, whether large packets get through, and whether your DNS resolver can reach a nameserver that only has IPv6.
Each test fetches a small file from a hostname set up to allow only one path: IPv4 only, IPv6 only, both, a path that only carries 1280-byte IPv6 packets, or a name that only an IPv6-capable DNS resolver can find. A fast failure on the IPv6 test means you have no IPv6. A slow one means you have IPv6 that does not work, which is the case worth fixing.
Six things. Whether you can reach a site over IPv4 only, over IPv6 only, and over both when a site offers both. Whether large replies reach you. Whether large IPv6 packets get through on a path that only carries small ones, which tests whether your network passes the ICMPv6 messages that make that work. And whether your DNS resolver can reach a nameserver that has only an IPv6 address.
Because they answer different questions. Almost every large site offers both IPv4 and IPv6, so the first score is what your connection is like today. The second is what happens when a site has no IPv4 at all. A connection with no IPv6 is perfectly fine on the first and scores zero on the second, and both of those are true.
No. Plenty of providers have not switched it on yet, and every site you use still offers IPv4. It becomes a problem only for services that are IPv6-only, which are rare today but increasingly common in mobile networks and newer hosting.
Your device has an IPv6 address and a route, so it tries IPv6 first, but the packets go nowhere. Browsers wait a moment and then fall back to IPv4, so most things still work, slowly. That delay happens on every site that offers both. It is worse than having no IPv6 at all, and the fix is usually a router restart or a call to your provider.
IPv6 routers never split packets. When one is too big for the next link, the router drops it and sends back a "Packet Too Big" message telling the sender to use smaller ones. If a firewall on your side blocks those messages, your device never finds out and keeps sending the big packet, so connections stall as soon as they have more than a little to say. Allowing ICMPv6 type 2 on the firewall fixes it.
Either. Browsers race the two and use whichever connects first, so a working IPv6 connection usually wins but not always. What matters is that the site loaded quickly; the family it used is informational.
It runs the same kind of tests, written independently, from our own network in Dhaka, Bangladesh, with a provider lookup on each address. If both say your IPv6 is broken, it is.
On What is my IP, which shows the address you are connected from with its reverse DNS, network and registry entry, and links each one into the rest of our tools.