On 17/03/2014 16:16, Ewan MacMahon wrote:
>> -----Original Message-----
>> From: Testbed Support for GridPP member institutes [mailto:TB-
>> [log in to unmask]] On Behalf Of Alessandra Forti
>>
>> Reading a bit more about it [1] what is not clear to me is if we really
>> need spaces bigger than /64 for single clusters.
>>
> I think you do, but possibly not all that much larger.
>
>> I mean /64 is only 1 subnet but has 18,446,744,073,709,551,616 IPv6
>> addresses.
>>
> This is true, but not really the key point - generally you don't get to
> use all of those addresses, what that gives you is the scope to allow
> your nodes to allocate their own addresses within that space. The
> assumption is that a /64 block is a single flat network. You can't
> subnet that space into (say) two /65 subnets without breaking things,
> or at the very least, breaking a lot of assumptions and making life
> difficult and complicated. It's more meaningful to think about that as,
> (as you say) one network.
> The 'lots of addresses' is essentially an
> implementation detail.
>
> In particular, I think you'd want at least two /64 subnets for a reasonable
> cluster
it didn't really seem an option in the discussion so far. either /64 or
/56 or /48. I went with the /48 NNW suggestion because I trusted they
would ask something sensible, but I guess they reason on another scale.
We don't need a /48 for sure.
> - one to use for stateless autoconfiguration, and one to use for
> manually assigned logical service addresses. A /56 allocation gives you
> 256 subnets, which is probably more than you'd need for something like a
> grid cluster,
> but is sensible for something like a department, where you
> might want to divide up (say) central services on one, client desktops
> on another, server management cards on another, or have divisions between
> different buildings or whatever.
>
>> One argument can be that we do fit the decirption if we don't go through
>> the University routers - Manchester doesn't for example - even though
>> administratively we belong to the University.
>>
> IPv6 addressing should normally reflect routing layouts, so if you're
> separately routed, I'd have thought you should have a separate address
> allocation. If you had a subnet of the University space, upstream routers
> would have a routing table entry for Manchester's address range that pointed
> to the university routers, and that would be wrong for you. It is possible
> to route something like that, but it's a bit awkward. However, I think you'd
> just need to have the discussion with Janet/NNW and see how they'd prefer
> to handle it in practice. It would/will be interesting to see what they say.
JANET refused this argument and told NNW we are part of the University
and we have to go through the University.
I guess administration counted more on this occasion.
>
>> I think that so far it was all very accademic and people are rushing to
>> get huge address spaces whether they need it or not.
>>
> I think there's an element of that, but it is important to think in terms
> of how many networks you have, not how many addresses, and there's also the
> argument that we can afford to hand out lots of space - as John notes,
> Janet have a /32 which is fairly spacious. If everyone were kept to the
> 'small institution' block of a /48, then that would allow 2^16 blocks. There's
> plenty of scope for giving major universities a more generous /44 without
> being too concerned about running out. That said, if everyone got that, we'd
> have a problem because there's only room for 2^12=4096 of those, and that's
> probably a bit tight - OK for universities, fatal if you start looping (say)
> schools in as well (a quick Google suggests that according to the DfE, there
> are ~24,000 schools in England). A mixture of allocation sizes at that
> level is probably sensible. At this point the balance being struck is between
> the liklihoods of running out of networks at the top level, and running out
> near the edges - it's worth avoiding giving people excessively stingy blocks
> and having to renumber them in the near future if you can avoid it.
but also excessive blocks would be just a waste IMO.
cheers
alessandra
>
> Ewan
|