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

Ray Soucy rps at
Mon Jan 24 14:53:32 CST 2011

Every time I see this question it' usually related to a fundamental
misunderstanding of IPv6 and the attempt to apply v4 logic to v6.

That said.  Any size prefix will likely work and is even permitted by
the RFC.  You do run the risk of encountering applications that assume
a 64-bit prefix length, though.  And you're often crippling the
advantages of IPv6.

But in terms of the "best practice" it is indeed 64-bit for every
network, with the option of 126-bit prefixes for link networks and
128-bit loopback addresses.

You should think of IPv6 as a 64-bit address that happens to include a
64-bit host identifier.

The entire point of IPv6 having a 128-bit address space was to
facilitate this and put an end to having to determine the network
prefix length based on an (often incorrect) estimation of the number
of hosts the network will need to accommodate.

Use of 126-bit prefixes for point-to-point connections (link networks)
is acceptable.  Use of 127-bit prefixes should be avoided as outlined
in RFC 3627.

So it really comes down to keeping it simple.

Remember, we're dealing with exponentials here.  A 64-bit address
space isn't twice as large as a 32-bit address space; it's roughly 4.2
billion times larger.  The 340 undecillion (that's 340 with 36 zeros
after it) unique identifiers available with a 128-bit address space is
blatantly excessive if you don't  factor in that the host segment is
always intended to be 64 of those bits.

So if conservation of address space isn't the logic behind using a
smaller prefix, then the question becomes what is?  Most people
tunnel-vision on the fact that Stateless Address Auto-Configuration
requires that a 64-bit prefix be advertised to work and assume that
the best way to disable it is to use a prefix-length other than 64.

While you could do this, a far better way would be to simply not
announce the prefix with the Autonomous bit set to true.  Every IPv6
implementation respects the value of the "A" bit on a prefix
advertisement and will not use Stateless configuration if it is not
true, just as outlined in the RFC.

Another thing to consider is that most processors today lack
operations for values that are larger than 64-bit.  By separating the
host and network segment at the 64-bit boundary you may be able to
take advantage of performance optimizations that make the distinction
between the two (and significantly reduce the cost of routing
decisions, contributing to lower latency).

Many cite concerns of potential DoS attacks by doing sweeps of IPv6
networks.  I don't think this will be a common or wide-spread problem.
 The general feeling is that there is simply too much address space
for it to be done in any reasonable amount of time, and there is
almost nothing to be gained from it.

But yes.  Basic NDP, routing, forwarding, etc. should work fine with
anything shorter than 126.  Just not really sure the logic behind
doing so.

On Mon, Jan 24, 2011 at 7:59 AM, Carlos Martinez-Cagnazzo
<carlosm3011 at> wrote:
> The subject says it all... anyone with experience with a setup like this ?
> I am particularly wondering about possible NDP breakage.
> cheers!
> Carlos
> --
> --
> =========================
> Carlos M. Martinez-Cagnazzo
> =========================

Ray Soucy

Epic Communications Specialist

Phone: +1 (207) 561-3526

Networkmaine, a Unit of the University of Maine System

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