IPv4 and IPv6 are two address systems used to identify devices and networks on the internet.
IPv4 uses dotted decimal addresses such as 8.8.8.8. It is still the most familiar format, but the number of available IPv4 addresses is limited. Many providers use NAT, carrier-grade NAT or shared addressing to stretch IPv4 capacity across many users.
IPv6 uses longer hexadecimal addresses such as 2001:db8::1. It provides a much larger address space and can reduce the need for address sharing. A dual-stack connection supports both IPv4 and IPv6, allowing websites and apps to choose the best available path.
DNS uses A records for IPv4 and AAAA records for IPv6. If one family fails while the other works, the issue may be DNS, routing, firewall rules or provider support.