<s>
Port	B-Protocol
triggering	I-Protocol
is	O
a	O
configuration	O
option	O
on	O
a	O
NAT-enabled	O
router	B-Protocol
that	O
controls	O
communication	O
between	O
internal	O
and	O
external	O
host	O
machines	O
in	O
an	O
IP	B-Protocol
network	I-Protocol
.	O
</s>
<s>
It	O
is	O
similar	O
to	O
port	B-Protocol
forwarding	I-Protocol
in	O
that	O
it	O
enables	O
incoming	O
traffic	O
to	O
be	O
forwarded	O
to	O
a	O
specific	O
internal	O
host	O
machine	O
,	O
although	O
the	O
forwarded	O
port	O
is	O
not	O
open	O
permanently	O
and	O
the	O
target	O
internal	O
host	O
machine	O
is	O
chosen	O
dynamically	O
.	O
</s>
<s>
When	O
two	O
networks	O
communicate	O
through	O
a	O
NAT-router	O
,	O
the	O
host	O
machines	O
on	O
the	O
internal	O
network	O
behave	O
as	O
if	O
they	O
have	O
the	O
IP	O
address	O
of	O
the	O
NAT-router	O
from	O
the	O
perspective	O
of	O
the	O
host	O
machines	O
on	O
the	O
external	O
network	O
.	O
</s>
<s>
This	O
is	O
because	O
the	O
connection	O
can	O
only	O
be	O
targeted	O
to	O
the	O
IP	O
of	O
the	O
NAT-router	O
,	O
since	O
the	O
internal	O
network	O
is	O
hidden	O
behind	O
NAT	B-Protocol
.	O
</s>
<s>
With	O
port	B-Protocol
triggering	I-Protocol
,	O
when	O
some	O
host	O
A	O
opens	O
a	O
connection	O
to	O
a	O
host	O
B	O
using	O
a	O
predefined	O
port	O
or	O
ports	O
,	O
then	O
all	O
incoming	O
traffic	O
that	O
the	O
router	B-Protocol
receives	O
on	O
some	O
predefined	O
port	O
or	O
ports	O
is	O
forwarded	O
to	O
host	O
A	O
.	O
</s>
<s>
Port	B-Protocol
triggering	I-Protocol
is	O
useful	O
for	O
network	O
applications	O
where	O
the	O
client	O
and	O
server	O
roles	O
must	O
be	O
switched	O
for	O
certain	O
tasks	O
,	O
such	O
as	O
authentication	O
for	O
IRC	B-Protocol
chat	I-Protocol
and	O
file	O
downloading	O
for	O
FTP	B-Protocol
file	O
sharing	O
.	O
</s>
<s>
As	O
an	O
example	O
of	O
how	O
port	B-Protocol
triggering	I-Protocol
operates	O
,	O
when	O
connecting	O
to	O
IRC	B-Protocol
(	O
Internet	B-Protocol
Relay	I-Protocol
Chat	I-Protocol
)	O
,	O
it	O
is	O
common	O
to	O
authenticate	O
a	O
username	O
with	O
the	O
Ident	O
protocol	O
via	O
port	O
113	O
.	O
</s>
<s>
When	O
connecting	O
to	O
IRC	B-Protocol
,	O
the	O
client	O
computer	O
typically	O
makes	O
an	O
outgoing	O
connection	O
on	O
port	O
6667	B-Protocol
(	O
or	O
any	O
port	O
in	O
the	O
range	O
6660	O
–	O
7000	O
)	O
,	O
causing	O
the	O
IRC	B-Protocol
server	O
to	O
attempt	O
to	O
verify	O
the	O
username	O
given	O
by	O
making	O
a	O
new	O
connection	O
back	O
to	O
the	O
client	O
computer	O
on	O
port	O
113	O
.	O
</s>
<s>
When	O
the	O
computer	O
is	O
behind	O
NAT	B-Protocol
,	O
the	O
NAT	B-Protocol
device	I-Protocol
silently	O
drops	O
this	O
connection	O
because	O
it	O
does	O
not	O
know	O
to	O
which	O
computer	O
behind	O
the	O
NAT	B-Protocol
it	O
should	O
send	O
the	O
request	O
to	O
connect	O
.	O
</s>
<s>
These	O
two	O
transport-level	O
connections	O
are	O
necessary	O
for	O
the	O
application-level	O
connection	O
to	O
the	O
IRC	B-Protocol
server	O
to	O
succeed	O
(	O
see	O
Internet	B-Protocol
protocol	I-Protocol
suite	I-Protocol
)	O
.	O
</s>
<s>
Since	O
the	O
second	O
TCP/IP	B-Protocol
connection	O
is	O
not	O
possible	O
,	O
the	O
attempted	O
connection	O
to	O
the	O
IRC	B-Protocol
server	O
will	O
fail	O
.	O
</s>
<s>
In	O
the	O
case	O
of	O
port	B-Protocol
triggering	I-Protocol
,	O
the	O
router	B-Protocol
is	O
configured	O
so	O
that	O
when	O
an	O
outbound	O
connection	O
is	O
established	O
on	O
any	O
port	O
from	O
6660	O
to	O
7000	O
,	O
it	O
should	O
allow	O
inbound	O
connections	O
to	O
that	O
particular	O
computer	O
on	O
port	O
113	O
.	O
</s>
<s>
This	O
gives	O
it	O
more	O
flexibility	O
than	O
static	O
port	B-Protocol
forwarding	I-Protocol
because	O
it	O
is	O
not	O
necessary	O
to	O
set	O
it	O
up	O
for	O
a	O
specific	O
address	O
on	O
your	O
network	O
,	O
allowing	O
multiple	O
clients	O
to	O
connect	O
to	O
IRC	B-Protocol
servers	O
through	O
the	O
NAT-router	O
.	O
</s>
<s>
Port	B-Protocol
triggering	I-Protocol
has	O
the	O
disadvantage	O
that	O
it	O
binds	O
the	O
triggered	O
port	O
to	O
a	O
single	O
client	O
at	O
a	O
time	O
.	O
</s>
<s>
As	O
long	O
as	O
the	O
port	O
is	O
bound	O
to	O
that	O
particular	O
client	O
,	O
port	B-Protocol
triggering	I-Protocol
is	O
effectively	O
unavailable	O
to	O
all	O
other	O
clients	O
.	O
</s>
<s>
In	O
FTP	B-Protocol
file	O
sharing	O
,	O
for	O
example	O
,	O
this	O
means	O
that	O
no	O
two	O
clients	O
can	O
download	O
files	O
from	O
an	O
FTP	B-Protocol
server	I-Protocol
running	O
on	O
"	O
active	O
mode	O
"	O
simultaneously	O
.	O
</s>
<s>
For	O
IRC	B-Protocol
,	O
even	O
though	O
the	O
authentication	O
step	O
happens	O
very	O
quickly	O
,	O
the	O
port	B-Protocol
triggering	I-Protocol
timeout	O
may	O
still	O
prevent	O
other	O
clients	O
from	O
logging	O
into	O
IRC	B-Protocol
servers	O
.	O
</s>
<s>
Port	B-Protocol
triggering	I-Protocol
is	O
unsuitable	O
for	O
servers	O
behind	O
a	O
NAT	B-Protocol
router	B-Protocol
because	O
it	O
relies	O
on	O
the	O
local	O
computer	O
to	O
make	O
an	O
outgoing	O
connection	O
before	O
it	O
can	O
receive	O
incoming	O
ones	O
.	O
</s>
<s>
On	O
some	O
routers	B-Protocol
it	O
is	O
possible	O
to	O
have	O
more	O
than	O
one	O
client	O
use	O
port	B-Protocol
triggering	I-Protocol
and	O
port	B-Protocol
forwarding	I-Protocol
,	O
but	O
not	O
simultaneously	O
.	O
</s>
