Biggest mistake for IPv6: It's not backwards compatible, developers admit

Jeroen Massar jeroen at
Wed Apr 1 13:47:34 CEST 2009

Benny Amorsen wrote:
> Fred Baker <fred at> writes:
>> Also, I think it is only fair to point out that they didn't have the
>> option of making it backwards compatible with IPv4; it's not that they
>> didn't, it's that they couldn't. How, precisely, would you make an
>> IPv4 packet that has longer addresses? IPv4 forces any change to the
>> header to become a new protocol.
> It doesn't, in hindsight.
> You could have added extra, optional fields to the IP header,

This new option field sets the version to 6, the NAT devices can parse
this new version field and.....

The moment you have to touch something to upgrade it, you need to
upgrade them all.

What you propose can also be accomplished by tunneling all IPv6 packets
inside IPv4 ala 6to4/proto41. The "Extra bit" then is that the protocol
field is set to 41 aka IPv6.

> Advantages: Routers don't need to be upgraded at all, except for NAT
> routers (and those are commonly upgraded already, unlike core routers).

If they are upgraded like that, you can also upgrade them to support
IPv6. Unfortunately those things get replaced, not upgraded.


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