I read many documents which are saying if you are in IPV4 network then you can not access IPV6 network, for example when I was at home I can not access some IPV6 website, such as:ipv6test.google.com.
My confusion is that what blocked they talk each other. When a IPV4 package goes out, the router routes it to IPV6 server, then the IPV6 server just route the package back and the router could find the client as well. Though the package header different, but the TCP protocol and data format are same, so what blocked the two IP versions talk each other?