About address allocating (IPv6, variable length SLA/prefixes?)

[Brian E Carpenter]

Wilfried,

Architecturally, IPv6 has the equivalent of variable length subnet masks
built in. There are really only two boundaries that are not flexible-
the boundary between the format prefix and the rest of the address, and the
/64 boundary. (The format prefix is in fact variable length, but it is
architecturally defined.) So any IGP or EGP design needs to be fully flexible
to the left of /64. Subnetting to the right of /64 would be tricky.

   Brian

"Wilfried Woeber, UniVie/ACOnet" wrote:
> 
>   Hi Francis et.al.,
> 
>   as I said a couple of days ago in Budapest, I would like to see an
>   explanation and/or review from the routing point of view.
> 
>   Judging from my (limited) knowledge about IPv6, going for a variable
>   length SLA field would either leave us with "wasted" address space (as
>   the network next door would be a different site and thus should have a
>   different NLA field anyway), or we would end up with a variable length
>   network prefix length (much like in the v4 environment), effectively
>   extending the NLA field into the SLA field.
> 
>   Doing so would probably require a cross-check against existing
>   IPv6-aware IGPs. That is where I would like to see input from the
>   routing camp(s).
> 
>   Regards,
>   Wilfried.
> ______________________________________________________________________
> From: Francis Dupont <Francis.Dupont@enst-bretagne.fr>
> To: itojun@iijlab.net
> CC: Haisang Wu <hswu@ns.6test.edu.cn>, 6bone@ISI.EDU
> Subject: Re: About address allocating
> Date: Sat, 20 May 2000 15:17:57 +0200
> 
>  In your previous mail you wrote:
> 
> 
>    >   hi, I have the following questions about address allocating:
>    >     I know SLA is /48, and interface ID should be 64 bits,
>    >   does it mean that the smallest unit when allocating address is /48?
>    >   In other words, if I allocate a /48 to a large university, could I
>    >   allocate a /48 to four middle schools, thus each middle school gets
>    >   an block less than /48, which is /50. Is this plan reasonable?
>    >
>    >=> we'd like to get a /48, ISPs would like to give a /64 to us:
>    > - /48 seems a bit too large for a default allocation size
>    > - /64 is unusable when you need subneting
>    >then the current idea, as presented yesterday here in Budapest
>    >at the RIPE meeting, is to introduce "small site" which get
>    >/56 (on byte boundary, large enough for up to 256 subnetworks or
>    >a few levels of hierarchy).
>    >Then /56 will become the default allocation size in RIR
>    >allocation & assignment document.
> 
>         I'm not sure if introducing "small sites" is a good thing...
>         when we switch ISP and they force me to switch from /48 to /56,
>         renumber becomes very hard.
> 
> => the idea is that it is easier for someone which needs a /48
> to deal with its ISP than for a common customer to fight in order to
> get a /48 because /64 is not enough: this is a compromise for common
> customers (ie you at home, IIJlab is strong enough to get a /x with x <= 48).
> I believe it is a good compromise...
> 
> Regards
> 
> Francis.Dupont@enst-bretagne.fr
> --------------------------------------------------------------------------------
>  _________________________________:_____________________________________
>   Wilfried Woeber                 : e-mail: Woeber@CC.UniVie.ac.at
>   UniVie Computer Center - ACOnet : Tel: +43 1 4277 - 140 33
>   Universitaetsstrasse 7          : Fax: +43 1 4277 - 9 140
>   A-1010 Vienna, Austria, Europe  : RIPE-DB: WW144, PGP keyID 0xF0ACB369
>  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~:~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

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Brian E Carpenter 
Program Director, Internet Standards & Technology, IBM 
On assignment for IBM at http://www.iCAIR.org 
Attend INET 2000: http://www.isoc.org/inet2000
Non-IBM email: brian@icair.org