<s>
X.121	B-Protocol
is	O
the	O
ITU-T	O
address	O
format	O
of	O
the	O
X.25	B-Protocol
protocol	I-Protocol
suite	I-Protocol
used	O
as	O
part	O
of	O
call	O
setup	O
to	O
establish	O
a	O
switched	O
virtual	B-Protocol
circuit	I-Protocol
between	O
Public	O
Data	O
Networks	O
(	O
PDNs	O
)	O
,	O
connecting	O
two	O
network	O
user	O
addresses	O
(	O
NUAs	O
)	O
.	O
</s>
<s>
It	O
consists	O
of	O
a	O
maximum	O
of	O
fourteen	O
binary-coded	O
decimal	O
digits	O
and	O
is	O
sent	O
over	O
the	O
Packet	B-Protocol
Layer	I-Protocol
Protocol	I-Protocol
(	O
PLP	O
)	O
after	O
the	O
packet	O
type	O
identifier	O
(	O
PTI	O
)	O
.	O
</s>
<s>
The	O
address	O
is	O
made	O
up	O
of	O
the	O
international	B-Protocol
data	I-Protocol
number	I-Protocol
(	O
IDN	O
)	O
,	O
which	O
consists	O
of	O
two	O
fields	O
:	O
the	O
4	O
digit	O
data	B-Protocol
network	I-Protocol
identification	I-Protocol
code	I-Protocol
(	O
DNIC	O
)	O
and	O
the	O
(	O
up	O
to	O
)	O
10	O
digit	O
national	B-Protocol
terminal	I-Protocol
number	I-Protocol
(	O
NTN	O
)	O
.	O
</s>
<s>
The	O
NTN	O
identifies	O
the	O
exact	O
network	O
device	O
(	O
DTE	O
,	O
data	O
terminal	O
equipment	O
)	O
in	O
the	O
packet-switched	B-Protocol
network	I-Protocol
(	O
PSN	O
)	O
and	O
is	O
often	O
provided	O
as	O
an	O
NUA	O
.	O
</s>
<s>
IPv4	O
addresses	O
can	O
be	O
mapped	O
to	O
X.121	B-Protocol
as	O
described	O
in	O
RFC	O
1236	O
.	O
</s>
<s>
The	O
14.0.0.0/8	O
block	O
used	O
to	O
be	O
reserved	O
for	O
X.121	B-Protocol
use	O
but	O
was	O
returned	O
to	O
IANA	O
in	O
2008	O
to	O
stave	O
off	O
IPv4	B-Protocol
address	I-Protocol
exhaustion	I-Protocol
.	O
</s>
