<s>
Carrier-grade	B-Protocol
NAT	I-Protocol
(	O
CGN	O
or	O
CGNAT	B-Protocol
)	O
,	O
also	O
known	O
as	O
large-scale	B-Protocol
NAT	I-Protocol
(	O
LSN	O
)	O
,	O
is	O
a	O
type	O
of	O
network	B-Protocol
address	I-Protocol
translation	I-Protocol
(	O
NAT	O
)	O
for	O
use	O
in	O
IPv4	B-Protocol
network	I-Protocol
design	O
.	O
</s>
<s>
With	O
CGNAT	B-Protocol
,	O
end	O
sites	O
,	O
in	O
particular	O
residential	O
networks	O
,	O
are	O
configured	O
with	O
private	B-Protocol
network	I-Protocol
addresses	O
that	O
are	O
translated	O
to	O
public	O
IPv4	B-Protocol
addresses	O
by	O
middlebox	O
network	B-Protocol
address	I-Protocol
translator	I-Protocol
devices	O
embedded	O
in	O
the	O
network	O
operator	O
's	O
network	O
,	O
permitting	O
the	O
sharing	O
of	O
small	O
pools	O
of	O
public	O
addresses	O
among	O
many	O
end	O
sites	O
.	O
</s>
<s>
Carrier-grade	B-Protocol
NAT	I-Protocol
is	O
often	O
used	O
for	O
mitigating	O
IPv4	B-Protocol
address	I-Protocol
exhaustion	I-Protocol
.	O
</s>
<s>
One	O
use	O
scenario	O
of	O
CGN	O
has	O
been	O
labeled	O
as	O
NAT444	B-Protocol
,	O
because	O
some	O
customer	O
connections	O
to	O
Internet	O
services	O
on	O
the	O
public	O
Internet	O
would	O
pass	O
through	O
three	O
different	O
IPv4	B-Protocol
addressing	O
domains	O
:	O
the	O
customer	O
's	O
own	O
private	B-Protocol
network	I-Protocol
,	O
the	O
carrier	O
's	O
private	B-Protocol
network	I-Protocol
and	O
the	O
public	O
Internet	O
.	O
</s>
<s>
Another	O
CGN	O
scenario	O
is	O
Dual-Stack	O
Lite	O
,	O
in	O
which	O
the	O
carrier	O
's	O
network	O
uses	O
IPv6	B-Protocol
and	O
thus	O
only	O
two	O
IPv4	B-Protocol
addressing	O
domains	O
are	O
needed	O
.	O
</s>
<s>
CGNAT	B-Protocol
techniques	O
were	O
first	O
used	O
in	O
2000	O
to	O
accommodate	O
the	O
immediate	O
need	O
for	O
large	O
numbers	O
of	O
IPv4	B-Protocol
addresses	O
in	O
General	O
Packet	O
Radio	O
Service	O
(	O
GPRS	O
)	O
deployments	O
of	O
mobile	O
networks	O
.	O
</s>
<s>
Estimated	O
CGNAT	B-Protocol
deployments	O
increased	O
from	O
1200	O
in	O
2014	O
to	O
3400	O
in	O
2016	O
,	O
with	O
28.85	O
%	O
of	O
the	O
studied	O
deployments	O
appearing	O
to	O
be	O
in	O
mobile	O
operator	O
networks	O
.	O
</s>
<s>
This	O
prompted	O
some	O
ISPs	O
to	O
develop	O
a	O
policy	O
within	O
the	O
American	O
Registry	O
for	O
Internet	O
Numbers	O
(	O
ARIN	O
)	O
to	O
allocate	O
new	O
private	B-Protocol
address	I-Protocol
space	O
for	O
CGNs	O
,	O
but	O
ARIN	O
deferred	O
to	O
the	O
IETF	O
before	O
implementing	O
the	O
policy	O
indicating	O
that	O
the	O
matter	O
was	O
not	O
a	O
typical	O
allocation	O
issue	O
but	O
a	O
reservation	O
of	O
addresses	O
for	O
technical	O
purposes	O
(	O
per	O
RFC	O
2860	O
)	O
.	O
</s>
<s>
Devices	O
evaluating	O
whether	O
an	O
IPv4	B-Protocol
address	O
is	O
public	O
must	O
be	O
updated	O
to	O
recognize	O
the	O
new	O
address	O
space	O
.	O
</s>
<s>
Allocating	O
more	O
private	O
IPv4	B-Protocol
address	O
space	O
for	O
NAT	B-Protocol
devices	I-Protocol
might	O
prolong	O
the	O
transition	O
to	O
IPv6	B-Protocol
.	O
</s>
<s>
Critics	O
of	O
carrier-grade	B-Protocol
NAT	I-Protocol
argue	O
the	O
following	O
aspects	O
:	O
</s>
<s>
Like	O
any	O
form	O
of	O
NAT	O
,	O
it	O
breaks	O
the	O
end-to-end	B-Application
principle	I-Application
.	O
</s>
<s>
It	O
has	O
significant	O
security	O
,	O
scalability	B-Architecture
,	O
and	O
reliability	B-Protocol
problems	O
,	O
by	O
virtue	O
of	O
being	O
stateful	B-Application
.	O
</s>
<s>
It	O
does	O
not	O
solve	O
the	O
IPv4	B-Protocol
address	I-Protocol
exhaustion	I-Protocol
problem	I-Protocol
when	O
a	O
public	O
IP	O
address	O
is	O
needed	O
,	O
such	O
as	O
in	O
web	O
hosting	O
.	O
</s>
<s>
Carrier-grade	B-Protocol
NAT	I-Protocol
usually	O
prevents	O
the	O
ISP	O
customers	O
from	O
using	O
port	B-Protocol
forwarding	I-Protocol
,	O
because	O
the	O
network	B-Protocol
address	I-Protocol
translation	I-Protocol
(	O
NAT	O
)	O
is	O
usually	O
implemented	O
by	O
mapping	O
ports	B-Protocol
of	O
the	O
NAT	B-Protocol
devices	I-Protocol
in	O
the	O
network	O
to	O
other	O
ports	B-Protocol
in	O
the	O
external	O
interface	O
.	O
</s>
<s>
This	O
is	O
done	O
so	O
the	O
router	B-Protocol
will	O
be	O
able	O
to	O
map	O
the	O
responses	O
to	O
the	O
correct	O
device	O
;	O
in	O
carrier-grade	B-Protocol
NAT	I-Protocol
networks	O
,	O
even	O
though	O
the	O
router	B-Protocol
at	O
the	O
consumer	O
end	O
might	O
be	O
configured	O
for	O
port	B-Protocol
forwarding	I-Protocol
,	O
the	O
"	O
master	O
router	B-Protocol
"	O
of	O
the	O
ISP	O
,	O
which	O
runs	O
the	O
CGN	O
,	O
will	O
block	O
this	O
port	B-Protocol
forwarding	I-Protocol
because	O
the	O
actual	O
port	O
would	O
not	O
be	O
the	O
port	O
configured	O
by	O
the	O
consumer	O
.	O
</s>
<s>
In	O
order	O
to	O
overcome	O
the	O
former	O
disadvantage	O
,	O
the	O
Port	B-Protocol
Control	I-Protocol
Protocol	I-Protocol
(	O
PCP	O
)	O
has	O
been	O
standardized	O
in	O
the	O
RFC	O
6887	O
.	O
</s>
<s>
If	O
that	O
user	O
happens	O
to	O
be	O
behind	O
carrier-grade	B-Protocol
NAT	I-Protocol
,	O
other	O
users	O
sharing	O
the	O
same	O
public	O
address	O
with	O
the	O
spammer	O
will	O
be	O
mistakenly	O
blocked	O
.	O
</s>
