An IPv4 address isn't A.B.C.D though. That's just a shorthand way for us write it down because those chunks are easier for us to deal with. An IPv4 address is a

An IPv4 address isn't A.B.C.D though. That's just a shorthand way for us write it down because those chunks are easier for us to deal with. An IPv4 address is an unsigned 32 bit integer. The address is not the real issue. An IPv4 header contains 32 bits for the source address and 32 bits for the destination address. That's what needed to be extended. IPv4 implementations expect the destination address to be 32 bits after the source address. Any fix you can think of to extend the address fields in the header will fall into the situation of having an additional protocol. Because changing the v4 header and calling it a v4 header would probably get your packet dropped as bad. Or have it being sent to somewhere else. Or some other undefined behavior. So your best option is to have it identify as a new version. Yes, v4 implementations will reject your packet, but that's what you want in this case.