Subnetting Practices

[Gert Doering]

Hi,

On Sat, Jul 14, 2007 at 11:16:56AM -0700, Seth Mattinen wrote:
> I'm inclined to use something *way* smaller (like a /126 since /127's 
> are bad) for router links. I thought the push behind IPv6 was because 
> we're running out of v4 space, and I see standard practice blowing a /64 
> on a link that'll never have more than 2 devices on it. Lots of stuff I 
> read encourages seemingly wasteful practices in v6 space as a good thing 
> and it confuses me.

Please try to get rid of your IPv4 mindset, and do a bit of math:

If each "end site" gets a /48, this leaves about 1000 /48s for each
possible inhabitant of the planet, out of FP001 (the first 8th of the
address space).  So this is something not overly wasteful.

Inside that /48, each end site has over 65.000 subnets.

So - if you're in serious risk of needing more than this number of
subnets, using /64 for every type of network (small, large, multiaccess
or not) might be a bit wasteful.  For a normal site, it's very unlikely
that it will ever need even a notable fraction of that number of subnets.

The focus of IPv6 is not on "squeeze out the least possible bit of 
address space" but on "have so much address space that operations can
be made much easier" - and having a /64 on every subnet is part of that.

(Personally, I would have preferred to use a /80, or a /96, or something
else which is still insanely huge - but in the end, as long as you don't
run out of bits in the network part, it doesn't really matter.  /64 has
the nice benefits of working with EUI-64 autoconfig, and that you can
use crypto-generated addresses, which require "enough bits" in the host
part).

And indeed, router vendors need to do a bit of homework to avoid being
an "easy target" DoS due to sequential IPv6 address scanning - but then,
they would need this for a network size of a /112 as well (still room
for 65.000 hosts, and potential ARP/ND requests).

Gert Doering
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