IPv6 can do that too though! 6in4 is exactly equivalent to the proposed EIP extension. EIP still splits the internet into the old legacy v4 internet and the new EIP internet that cannot communicate with the old internet, since while you can send packets to a v4-only host it won't know what to do with it. You need to preserve this extra information - and IPv4 simply cannot do that. You cannot fit more than 32 bit of information into 32 bit..