<s>
In	O
order	O
to	O
ensure	O
proper	O
working	O
of	O
carrier-grade	B-Protocol
NAT	I-Protocol
(	O
CGN	O
)	O
,	O
and	O
,	O
by	O
doing	O
so	O
,	O
alleviating	O
the	O
demand	O
for	O
the	O
last	O
remaining	B-Protocol
IPv4	I-Protocol
addresses	I-Protocol
,	O
a	O
size	O
IPv4	B-Protocol
address	I-Protocol
block	O
was	O
assigned	O
by	O
Internet	O
Assigned	O
Numbers	O
Authority	O
(	O
IANA	O
)	O
to	O
be	O
used	O
as	O
shared	O
address	O
space	O
.	O
</s>
<s>
This	O
block	O
of	O
addresses	O
is	O
specifically	O
meant	O
to	O
be	O
used	O
by	O
Internet	O
service	O
providers	O
(	O
or	O
ISPs	O
)	O
that	O
implement	O
carrier-grade	B-Protocol
NAT	I-Protocol
,	O
to	O
connect	O
their	O
customer-premises	O
equipment	O
(	O
CPE	O
)	O
to	O
their	O
core	B-Protocol
routers	I-Protocol
.	O
</s>
<s>
The	O
reason	O
is	O
that	O
routing	B-Protocol
and	O
network	B-Protocol
address	I-Protocol
translation	I-Protocol
(	O
NAT	O
)	O
will	O
not	O
work	O
if	O
the	O
same	O
address	O
ranges	O
occur	O
on	O
both	O
inside	O
and	O
outside	O
network	O
interfaces	O
.	O
</s>
<s>
In	O
2012	O
,	O
the	O
IETF	O
defined	O
a	O
Shared	O
Address	O
Space	O
for	O
use	O
in	O
ISP	O
CGN	O
deployments	O
and	O
NAT	B-Protocol
devices	I-Protocol
that	O
can	O
handle	O
the	O
same	O
addresses	O
occurring	O
both	O
on	O
inbound	O
and	O
outbound	O
interfaces	O
.	O
</s>
<s>
The	O
use	O
of	O
shared	O
address	O
space	O
is	O
one	O
of	O
the	O
various	O
methods	O
to	O
allow	O
transition	B-Protocol
from	O
IPv4	O
to	O
IPv6	B-Protocol
.	O
</s>
<s>
A	O
common	O
practice	O
is	O
to	O
give	O
CPEs	O
a	O
unique	O
IPv4	B-Protocol
address	I-Protocol
on	O
their	O
Internet-facing	O
interface	O
and	O
use	O
NAT	O
to	O
hide	O
all	O
addresses	O
on	O
the	O
home	O
LAN	B-General_Concept
.	O
</s>
<s>
Since	O
the	O
pool	O
of	O
available	O
public	O
IPv4	O
addresses	O
is	O
depleted	B-Protocol
,	O
it	O
is	O
no	O
longer	O
possible	O
for	O
most	O
ISPs	O
to	O
assign	O
unique	O
IPv4	O
addresses	O
to	O
CPEs	O
,	O
because	O
there	O
are	O
none	O
left	O
to	O
them	O
to	O
acquire	O
.	O
</s>
<s>
Instead	O
,	O
an	O
address	O
in	O
the	O
range	O
is	O
assigned	O
on	O
the	O
CPE	O
's	O
Internet-facing	O
interface	O
,	O
and	O
this	O
address	O
is	O
translated	O
again	O
to	O
one	O
of	O
the	O
public	O
IPv4	O
addresses	O
of	O
the	O
ISP	O
's	O
core	B-Protocol
routers	I-Protocol
.	O
</s>
<s>
This	O
scheme	O
hides	O
a	O
large	O
number	O
of	O
IP	O
addresses	O
behind	O
a	O
small	O
set	O
of	O
public	O
addresses	O
,	O
the	O
same	O
way	O
the	O
CPE	O
does	O
this	O
locally	O
,	O
slowing	O
down	O
the	O
rate	O
IPv4	O
addresses	O
are	O
depleted	B-Protocol
.	O
</s>
<s>
In	O
BIND	B-Protocol
,	O
empty	O
reverse	O
mapping	O
zones	O
for	O
till	O
(	O
64	O
zones	O
in	O
total	O
)	O
are	O
automatically	O
created	O
in	O
the	O
'	O
internal	O
 '	O
view	O
,	O
if	O
not	O
configured	O
otherwise	O
.	O
</s>
