When starting the java server, if IPv6 is available the server will listen on the IPv6 address and ignore IPv4 unless you set the `server-ip` in server.properties. This only allows you to choose either IPv4 or IPv6 and not both (also called dual stacking). Having a dual stack is probably the best thing during the transition over to IPv6 so then those with IPv6 connections can connect to servers with IPv6 and those without can still use IPv4