There are a variety of "IPvX over Y" RFCs. That is correct. You are just reading it a bit wrong. And note that Marc Blanchet is one of the few people who actually knows what he is talking of when concerning IPv6. I don't have that book in my little collection though. The steps for IPv6 over Ethernet are: - Parse the Ethernet Header - Look at the Ether Type: if 0x0800 pass it to the IPv4 stack, if 0x08dd pass it to the IPv6 stack, etc Then the IP stack, which could be a combined IPv4/IPv6 stack or a split IPv4/IPv6 stack kicks in: - if version is 4 parse the rest as IPv4 - if version is 6 parse the rest as IPv6 Thus there is nothing wrong with that text, the check just happens on two layers. Even AICCU does this for AYIYA, and sixxsd does it also for proto-41: it first checks Ethernet or AYIYA what it states to be inside the packet, and then checks the version number if that is really in there. IP can be carried in anything else, this way one can do fast switching on the Ethernet level and one could use IP outside of the Ethernet header or anything else and still know what it is. It is 'redundant' but that gives an extra level of verification too, as one should never have an Ethernet frame stating IPv6 while IPv4 is in there for instance.