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

[Mark Smith <nanog () 85d5b20a518b8f6864949bd940457dc124746ddc nosense org>]

On Wed, 20 Oct 2010 14:48:47 -0700
Jeroen van Aart <jeroen () mompl net> wrote:

> <IPv6 newbie>
>
> 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 a pseudo random number, not follow bad examples. Where are these
examples? I'd be curious as to what they say regarding why they haven't
followed the pseudo random number requirement.

> 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.

DNS (including dynamic DNS, multicast DNS, and DNS service discovery) is
intended to be used far more often in IPv6 than it was in IPv4. It was
never going to be that possible to expand the size of the address space
significantly without trading off 'rememberability'.


The best way to understand ULAs is to read the RFC. It'd probably take
about 15 to 20 minutes, and is quite readable (as are most if not all
RFCs)

Unique Local IPv6 Unicast Addresses
http://tools.ietf.org/rfc/rfc4193.txt

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Regards,
Mark.