Using IPv6 with prefixes shorter than a /64 on a LAN

Mark Andrews marka at
Tue Jan 25 15:57:16 CST 2011

In message <op.vpvur91htfhldh at>, "Ricky Beam" writes:
> On Mon, 24 Jan 2011 19:46:19 -0500, Owen DeLong <owen at> wrote:
> > Dude... In IPv6, there are 18,446,744,073,709,551,616 /64s.
> Those who don't learn from history are doomed to repeat it.
> "Dude, there are 256 /8 in IPv4."
> "640k ought to be enough for anyone."

And there was a time that IPv6 address might have been 64 bits in
length and rather than having to managed 64 bits of addressing like
we were managing IPv4 (smallest sized networks that would do the job)
it was decided to double the address length and just manage networks
and their aggregation rather than networks and their sizes.

That was a deliberate decision.   We are still packing networks
reasonable densely especially anything /48 or shorter.

> People can mismange anything into oblivion.  IPv6 will end up the same  
> mess IPv4 has become. (granted, it should take more than 30 years this  
> time.)
> > The largest ISPs have thousands (not tens of thousands) of  
> > point-to-point links.
> Having worked for small ISPs, I can count over 10k ptp links.  That number  
> goes way up when you count dialup and DSL.
> >>> You should think of IPv6 as a 64-bit address that happens to include a
> >>> 64-bit host identifier.
> >>
> >> No, you should not.  That underminds the fundamental concept of IPv6  
> >> being *classless*.  And it will lead to idiots writing broken  
> >> applications and protocols assuming that to be true.
> >>
> > True, but, in terms of deploying networks, unless you have a really good
> > reason not to, it is best to use /64 for all segments.
> Again, the only reason for this /64 class boundry is SLAAC.  The network  
> is still 128 bits; you still have to pay attention to ALL of those bits.
> (Remember, SLAAC started out as a /80.)
> > Blindly, no. However, it's not impractical to implement fast path  
> > switching that
> > handles things on /64s and push anything that requires something else
> > to the slow path.
> Any router that does CPU switching is already trash.  High speed, low  
> latency routing and switches is done in silicon (fpga's); it is not hoised  
> to a general purpose CPU.
> For consumer devices, (almost) everything is done by the CPU to make it  
> cheap. (some actually have tiny single chip switches in there.)
> --Ricky
Mark Andrews, ISC
1 Seymour St., Dundas Valley, NSW 2117, Australia
PHONE: +61 2 9871 4742                 INTERNET: marka at

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