On Thu, 11 Aug 2011, Robin Stevens wrote:

> Interesting.  We successfully argued that a /48 was insufficient to 
> avoid a fragmented internal allocation strategy, and I believe Cambridge 
> have subsequently done the same - both of us now have /44s, but perhaps 
> we should have pushed for more :-)

Indeed we did, but the policies under which the Cambridge /44 was 
allocated are quite different from the Oxford one.  I believe the rules 
had changed twice between the two applications!

The main beef was around subnets and internal administration - as I stated 
at my Networkshop talk: we (the central Computing Service) do not act as a 
single central authority on how all subnetting is carried out within the 
university, but act more like an ISP and route blocks of addresses down to 
the colleges and many of the larger internal departments.  When you get 
into that situation, you inevitably waste address space because you grant 
a block that you hope is large enough (but not too large) for their needs 
for the next 10-20 years (perhaps).

Coupled with DNS delegation (which we don't generally do, but I can see it 
happening with increased dynamic usage) then it's much nicer to delegate 
on a nibble boundary, so you're stuck with a /56 or /60.  Even if you 
allocate something smaller, you want to feel fairly confident you won't 
have to give them another 16 subnets (/60) in the future, perhaps.

When we started working all that out, giving internal institutions /56s 
would fit in a /48 but got a bit tight before anything new had come along 
(160-200x /56s = ~65%).  The next /48 up from ours was already allocated 
so there wouldn't be any room to expand, if we ever needed it.

We had quite a bit of trouble getting RIPE to understand the internal 
devolved nature of the university and affiliated institutions - yes 65K 
subnets is enough, but it's difficult to decide on what size block you 
allocate any particular department/college.  One of the nice things about 
IPv6 is supposed to that you can use all this address space to organise 
things on nice boundaries and not grow things piecemeal.

[Or, if you like, make all the mistakes we made with IPv4 back in the 
early 90s all over again.]


> Given that some ISPs are giving out /48s to domestic customers, then it 
> doesn't seem so unreasonable to grant a larger allocation to a large 
> university, with many departments and colleges, each of whom may require 
> multiple internal networks for various groups and applications.

I think the updated RFC that was published in late March / early April 
[the one Rob quoted] revises that and, I'm told, that Andrews and Arnold 
no longer offer a /48 by default to new customers.  I can see that a home 
user might need more than one subnet, but 65K seems silly (especially if 
you consider an ISP having 65K customers would then have used up their 
/32).


All that said, we're moving our office network from 2001:630:200:8100::/64 
to 2001:630:212:100::/64 this week and demonstrating just how easy prefix 
migration is on a subnet!

   - Bob


-- 
  Bob Franklin <[log in to unmask]>              +44 1223 748479
  Network Division, University of Cambridge Computing Service