RE: site-local != NAT

Margaret Wasserman <mrw@windriver.com> · Wed, 30 Apr 2003 14:28:16 -0400

Hi Tony,

At 09:44 AM 4/30/2003 -0700, Tony Hain wrote:
What many are missing here is that this is not about 1918 style
addressing. This is about the fact that addresses do not have the same
visibility and accessibility throughout the network. This operational
reality causes the affect we have labeled scoping.

There are three ways in which a particular address can be non-global:

        - Unreachable:  The address can be unreachable from some
                portions of the network (due to filtering, network
                failures, etc.).

        - Not Globally Routed:  There can be no route to the address
                from some portions of the network.

        - Not Globally Unique (AKA Ambiguous): A single address can
                indicate one node when it is used in one part of
                the network, and a different node when it is used
                in other parts of the

It is an inevitable fact of physics that some addresses will be
unreachable from some parts of the network.  The widespread
use of filtering/firewalls makes this both intentional and common.
Applications must deal with this (either well or badly), because
there is no other choice.

If the only way to get a globally routable address is to get it
from your provider, people may choose to use addresses that are
not globally routed for some purposes, such as convenience or
provider-independence.

I have not heard any compelling operational requirement for
ambiguous addresses in IPv6.

The IPv6 scoped addressing architecture defines addresses that
have all three of these properties, and this is what I hear when
you say "scoped addressing".  There could be types of local
addressing, though, that don't have all of these properties.

If you are using the term "scoped" addressing to refer to
something other than the type of addressing defined in the
IPv6 scoped addressing architecture, could you define your
term?

Thanks,
Margaret

P.S. There were some proposals to remove (or lessen the
likelihood of) ambiguous site-local addresses, but none of
those proposals removed the restrictions from the IPv6 scoped
addressing architecture that are only required because addresses
are ambiguous (sites can't be nested or overlap, zone IDs are
needed to disambiguate, etc.).