Why ULA: low collision chance (Was: IPv6 fc00::/7 — Unique local addresses)

Thu Oct 21 11:47:13 UTC 2010

On 2010-10-21 13:33, Ray Soucy wrote:
[..]
> People may throw a fit at this, but as far as I'm concerned FD00::/8
> will never leave the edge of our network (we null route ULA space
> before it can leak out, just like you would with RFC1918 space).  So
> you can pretty much use it has you see fit.  If you want to keep your
> ULA space short there is nothing stopping you from using something
> like FD00::1 as a valid address.

And then your company gets bought and you need to merge networks, that
is: renumber as they picked the same prefix.

There is nothing wrong with RFC1918 per se, the big problem with it is
that everybody else uses the same prefix, thus when you need to merge
two networks you have collisions.

I at one time also though that 'merging networks' and 'renumbering' is
easy, till I heard stories from folks who where doing that for really
large networks, who basically told that they where introducing 7+ layers
of NAT to solve that issue, as renumbering is simply not doable if you
have a global organization and if you are merging things like banks, for
some magic reason they want to be able to talk to eachother.

That is why there is ULA:
  low chance of collisions if one wants to stay in the RFC1918 mindset.

And if you want a guarantee of no collisions:
  go to your favorite RIR and get a prefix from them.

Greets,
 Jeroen