The short answer
Your public IPv4 address is the internet-facing address used for this family-specific test. It may be shared with other customers through carrier-grade NAT and it can change when you reconnect or switch networks.
Public IPv4 versus private IPv4
A public IPv4 address is routable on the internet and is visible to the services you connect to. Private addresses such as 192.168.x.x, 10.x.x.x and 172.16–31.x.x are used inside local networks and are not routed directly across the public internet.
Your router usually translates private device addresses to one public address using NAT. Mobile and some home providers add another provider-level translation layer, which means many customers can share the same public IPv4.
Why your IPv4 address changes
Most consumer connections use dynamically assigned addresses. Restarting a router, reconnecting mobile data, moving between Wi-Fi networks or enabling a VPN can change the public address websites see.
A VPN replaces your normal public IP with the VPN server’s exit address for traffic routed through it. That can hide your home IP from the destination, but the VPN provider becomes an intermediary and the browser may still expose other identifying signals.
Why IPv4 may not be detected
Some modern networks are IPv6-only or use translation to reach IPv4 services. A VPN, firewall, DNS filter, privacy extension or content blocker can also prevent the IPv4-only request from completing even though ordinary websites still load.