"4.3. IPv6-Only Core Network
An emerging deployment model uses IPv6 as the dominant protocol at
a
service provider network, and tunnels IPv4 through this network in
a
manner converse to the one described in the previous section.
There
are several motivations for choosing this deployment
model:
o There may not be enough
public or private IPv4 addresses to
support network
management functions in an end-to-end
fashion,
without
segmenting the network into small parts with
overlapping
address
space.
o IPv4 address sharing among
subscribers may involve new address
translation
nodes within the service provider's network. IPv6
can
be used to reach
these nodes. Normal IPv4 routing is
insufficient
for this
purpose, as the same addresses would be used in
several
parts of the
network.
o It may be simpler for the
service provider to employ a single-
version
network.
The recommended tool for this model is Dual Stack
Lite
[I-D.ietf-softwire-dual-stack-lite]. Dual Stack Lite provides
both
relief for IPv4 address shortage and makes forward progress on
IPv6
deployment, by moving service provider networks and IPv4 traffic
over
IPv6. Given this IPv6
connectivity, as a side-effect it becomes easy
to provide IPv6 connectivity all the way to the end users."