Where is the block which caused IPV4 client can not access IPV6 Server

Question

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?

Answer 1

My confusion is that what blocked they talk each other.

Nothing is blocking; they are just two completely different, incompatible protocols.

When a IPV4 package goes out, the router routes it to IPV6 server...

That cannot happen. An IPv4 packet has a 32-bit IPv4 destination address, so you cannot send an IPv4 packet to a destination with a128-bit IPv6 address. The IPv6 addresses do not fit in an IPv4 packet header.

...then the IPV6 server just route the package back and the router could find the client as well.

Again, that cannot happen because the addressing is incompatible.

Though the package header different, but the TCP protocol and data format are same, so what blocked the two IP versions talk each other?

Nothing is blocked, but the addressing simply will not allow a packet from one protocol to be addressed to a destination with a different protocol. TCP was also ported to IPX, but IPX and IP are incompatible protocols that simply cannot communicate. The same holds for IPv4 and IPv6.