BBC News - 'Fresh proposals' planned over cyber-monitoring

Andrew Cormack Andrew.Cormack at
Mon May 13 10:09:51 BST 2013

> -----Original Message-----
> From: ukcrypto-bounces at [mailto:ukcrypto-
> bounces at] On Behalf Of Ian Batten
> Sent: 12 May 2013 23:31
> To: UK Cryptography Policy Discussion Group
> Subject: Re: BBC News - 'Fresh proposals' planned over cyber-monitoring
> On 11 May 2013, at 17:26, Roland Perry <lists at>
> wrote:
> > In article <EADEBDCB-9FBA-4EAA-ADC8-444B93CAD478 at>, Ian
> Batten <igb at> writes
> >>> Basically, the people who specified IPv6 screwed up, big time, in
> not making it backwards compatible. They've got all sorts of excuses,
> that
> >>> only geeks who understand products, but not product management,
> would relate to.
> >>
> >> It would be interesting to understand what such a protocol would
> look like.  The basic problem surely is that if you have two address
> spaces,
> >> one larger than the other, you can't have a 1:1 mapping between the
> two (and to do so would defeat the object of making the address space
> >> larger).
> >
> > All that was necessary was making the bottom 0.1% (or whatever) of
> IPv6 map onto the old IPv4 space.
> If only someone had thought to insert a paragraph into RFC 2373,
> fifteen years ago, defining an IPv6 address whose first 80 bits is
> zero, following by 16 bits of one, as being an IPv4 address mapping
> into the IPv6 space, and thought to call them, oh, I don't know, an
> "IPv4-mapped IPv6 address".  They could have inserted it between
> sections 2.5.3 and 2.5.5, perhaps, and made things much easier.
> ian

If I remember correctly (and it was a *very* long time ago) one of the aims of v6 was to reduce the size of routing tables by bringing a bit of structure to address allocations. Doesn't mapping v4 addresses into part of the v6 space mean that the v6 table has to be at least as big as the v4 one from day one?

That may be less of an issue nowadays (the v6 designers presumably had in mind the memory limits/costs of routers twenty years ago) but it may be one reason why they chose a clean break.


Andrew Cormack
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