How the test works
Your browser requests a test address that has only an IPv4 (A) record. The server answers with the address it saw. If it answers, IPv4 works end to end: your device, router, provider and DNS. The time shown is the full request including connection setup.
When IPv4 fails
An IPv6-only network with no translation cannot reach IPv4-only sites. Most IPv6-only mobile networks run NAT64 and DNS64 so IPv4 sites still work; the test then succeeds through the translator.
A failed test can also mean a firewall, a captive portal (hotel or airport Wi-Fi waiting for you to sign in) or a VPN that blocks the request.
IPv4 address exhaustion
The pool of 4.3 billion IPv4 addresses ran out at the regional registries between 2011 and 2020. Providers now share addresses with carrier-grade NAT and buy blocks on the transfer market, which is why IPv6 is being rolled out.
Questions
- Why do I see an address starting with 100.64?
- Not here: that range only appears inside a provider's network. If your router's WAN address is in 100.64.0.0/10, your provider uses carrier-grade NAT and your public address is shared.