v6 subnet size for DSL & leased line customers

Mark Smith nanog at 85d5b20a518b8f6864949bd940457dc124746ddc.nosense.org
Sun Dec 23 07:55:23 UTC 2007

On Sat, 22 Dec 2007 12:53:52 -0800
"Christopher Morrow" <morrowc.lists at gmail.com> wrote:

> On Dec 22, 2007 12:23 PM, Ross Vandegrift <ross at kallisti.us> wrote:
> >
> > On Fri, Dec 21, 2007 at 01:33:15PM -0500, Deepak Jain wrote:
> > > For example... Within one's own network (or subnet if you will) we can
> > > absorb all the concepts of V4 today and have lots of space available.
> > > For example... for the DMZ of a business... Why not give them 6 bits
> > > (/122?) are we anticipating topology differences UPSTREAM from the
> > > customers that can take advantage of subnet differences between /64 and
> > > /56 ?
> >
> > I am confused on this point as well.  IPv6 documents seem to assume
> > that because auto-discovery on a LAN uses a /64, you always have to
> > use a /64 global-scope subnet.  I don't see any technical issues that
> > require this though.  ICMPv6 is capable of passing info on prefixes of
> > any length -  prefix length is a plain old 8bit field.
> >
> Uhm, so sure the spec might be able to do something different than /64
> but most equipment I've used only does auto-conf if the prefix is a
> /64 :( Somewhere along the path to ipng we got reverted to classful
> addressing again :(

Not really. Classful IPv4 defined both an addressing structure *and* an
agorithm to match destinations against the route table entries (i.e.
classful forwarding won't match on a default route if the router knows
at least one prefix within a classful network).

IPv6 uses the longest match rule regardless of any addressing
structure, and only uses structure for a few portions of the total
IPv6 address space, for the operation of things like DHCPv6 and address
autoconfiguration. A change in IPv6 addressing structure won't involve
a change in the route table matching algorithm.



        "Sheep are slow and tasty, and therefore must remain constantly
                                   - Bruce Schneier, "Beyond Fear"

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