IPv6 fc00::/7 — Unique local addresses

Thu Oct 21 05:28:52 UTC 2010

In message <4CBFC1D0.60808 at apolix.co.za>, Graham Beneke writes:
> On 21/10/2010 02:41, Owen DeLong wrote:
> > On Oct 20, 2010, at 5:21 PM, Jeroen van Aart wrote:
> >> Someone advised me to use GUA instead of ULA. But since for my purposes th
> is is used for an IPv6 LAN would ULA not be the better choice?
> >>
> > IMHO, no. There's no disadvantage to using GUA and I personally don't think
>  ULA really serves a purpose. If you want to later connect this
> > LAN to the internet or something that connects to something that connects t
> o something that connects to the internet or whatever, GUA provides
> > the following advantages:
> > 	+	Guaranteed uniqueness (not just statistically probable uniquene
> ss)
> > 	+	You can route it if you later desire to
> >
> > Since ULA offers no real advantages, I don't really see the point.
> 
> Someone insisted to me yesterday the RFC1918-like address space was the 
> only way to provide a 'friendly' place for people to start their journey 
> in playing with IPv6. I think that the idea of real routable IPs on a 
> lab network daunts many people.
> 
> I've been down the road with ULA a few years back and I have to agree 
> with Owen - rather just do it on GUA.

Your throwing the baby out with the bath water here.

ULA, by itself, is a painful especially when you have global IPv4
reachability as you end up with lots of timeouts.  This is similar
to have a bad 6to4 upsteam link.  Just don't go there.

ULA + PA works and provides stable internal addresses when your
upstream link in down the same way as RFC 1918 provides stable
internal addressing for IPv4 when your upstream link is down.

You talk to the world using PA addresses, directly for IPv6 and
indirectly via PNAT for IPv4.  These can change over time.

Similarly, ULA + 6to4 works well provided the 6to4 works when you
are connected.  When your IPv4 connection is renumbered you have a
new external addresses but the internal addresses stay the same.

> I was adding IPv6 to a fairly large experimental network and started 
> using ULA. The local NREN then invited me to peer with them but I 
> couldn't announce my ULA to them. They are running a 'public Internet' 
> network and have a backbone that will just filter them.
> 
> I think that the biggest thing that trips people up is that they think 
> that they'll just fix-it-with-NAT to get onto the GUA Internet. Getting 
> your own GUA from an RIR isn't tough - rather just do it.

If your big enough to get your own GUA and have the dollars to get
it routed then do that.  If you are forced to use PA (think home
networks) then having a ULA prefix as well is a good thing.

Mark
-- 
Mark Andrews, ISC
1 Seymour St., Dundas Valley, NSW 2117, Australia
PHONE: +61 2 9871 4742                 INTERNET: marka at isc.org