Re: IPv6 fc00::/7 — Unique local addresses

Thu Oct 21 00:10:30 UTC 2010

Hi Jeroen,

On Thu, Oct 21, 2010 at 8:48 AM, Jeroen van Aart <jeroen at mompl.net> wrote:
> According to http://en.wikipedia.org/wiki/IPv6_address#Special_addresses an
> fc00::/7 address includes a 40-bit pseudo random number:
>
> "fc00::/7 — Unique local addresses (ULA's) are intended for local
> communication. They are routable only within a set of cooperating sites
> (analogous to the private address ranges 10/8, 172.16/12, and 192.168/16 of
> IPv4).[12] The addresses include a 40-bit pseudorandom number in the routing
> prefix intended to minimize the risk of conflicts if sites merge or packets
> are misrouted into the Internet. Despite the restricted, local usage of
> these addresses, their address scope is global, i.e. they are expected to be
> globally unique."
>
> I am trying to set up a local IPv6 network and am curious why all the
> examples I come accross do not seem to use the 40-bit pseudorandom number?
> What should I do? Use something like fd00::1234, or incorporate something
> like the interface's MAC address into the address? It'd make the address
> quite unreadable though.

RFC4193 specifies a suggested algorithm to do it:
http://tools.ietf.org/html/rfc4193#section-3.2.2

The section 3.2.1 also states that
"Locally assigned Global IDs MUST be generated with a pseudo-random
   algorithm consistent with [RANDOM].  Section 3.2.2 describes a
   suggested algorithm.  It is important that all sites generating
   Global IDs use a functionally similar algorithm to ensure there is a
   high probability of uniqueness."

I'm not sure where did you find the examples you've mentioned. If it's
just a documentation example - seems to be fine. If someone is doing
it in real networks - that's just not right..

-- 
SY, Jen Linkova aka Furry