ISP customer assignments

Thu Oct 15 23:51:21 UTC 2009

> Of course, with IPv4, you never assigned a large enough block to begin
> with that would anticipate all growth, so routing additional blocks was
> a lot easier than changing blocks, cleaner than secondary IPs
> multiplying like crazy, etc., etc.  None of that would be an issue with
> a single /64.

You've hit on the key difference of IPv6. With IPv6 you should design
your network
so that it can grow for a long time without increasing the address
block sizes anywhere.
A /64 will work for even the biggest subnets. A /48 will do for for
very, very big sites.
And only the largest ISPs will outgrow a /32 allocation. If you assign a /48 to
a data center site, then when you subnet it, try to maintain that growth ability
if you can. Don't skimp on address block sizes unless you are backed into
a corner for technical or business reasons.

--Michael Dillon