<s>
In	O
security	O
parlance	O
,	O
the	O
term	O
open	B-Protocol
port	I-Protocol
is	O
used	O
to	O
mean	O
a	O
TCP	B-Protocol
or	O
UDP	B-Protocol
port	I-Protocol
number	O
that	O
is	O
configured	O
to	O
accept	O
packets	B-Protocol
.	O
</s>
<s>
In	O
contrast	O
,	O
a	O
port	O
which	O
rejects	O
connections	O
or	O
ignores	O
all	O
packets	B-Protocol
directed	O
at	O
it	O
is	O
called	O
a	O
closed	B-Protocol
port	I-Protocol
.	O
</s>
<s>
Ports	O
are	O
an	O
integral	O
part	O
of	O
the	O
Internet	O
's	O
communication	B-Protocol
model	I-Protocol
—	O
they	O
are	O
the	O
channel	O
through	O
which	O
applications	O
on	O
the	O
client	B-Protocol
computer	I-Protocol
can	O
reach	O
the	O
software	O
on	O
the	O
server	B-Application
.	O
</s>
<s>
Services	O
,	O
such	O
as	O
web	O
pages	O
or	O
FTP	B-Protocol
,	O
require	O
their	O
respective	O
ports	O
to	O
be	O
"	O
open	O
"	O
on	O
the	O
server	B-Application
in	O
order	O
to	O
be	O
publicly	O
reachable	O
.	O
</s>
<s>
There	O
needs	O
to	O
be	O
an	O
application	O
(	O
service	O
)	O
listening	O
on	O
that	O
port	O
,	O
accepting	O
the	O
incoming	O
packets	B-Protocol
and	O
processing	O
them	O
.	O
</s>
<s>
If	O
there	O
is	O
no	O
application	O
listening	O
on	O
a	O
port	O
,	O
incoming	O
packets	B-Protocol
to	O
that	O
port	O
will	O
simply	O
be	O
rejected	O
by	O
the	O
computer	O
's	O
operating	O
system	O
.	O
</s>
<s>
Ports	O
can	O
be	O
"	O
closed	O
"	O
(	O
in	O
this	O
context	O
,	O
filtered	O
)	O
through	O
the	O
use	O
of	O
a	O
firewall	B-Application
.	O
</s>
<s>
The	O
firewall	B-Application
will	O
filter	O
incoming	O
packets	B-Protocol
,	O
only	O
letting	O
through	O
those	O
packets	B-Protocol
for	O
which	O
it	O
has	O
been	O
configured	O
.	O
</s>
<s>
Packets	B-Protocol
directed	O
at	O
a	O
port	O
which	O
the	O
firewall	B-Application
is	O
configured	O
to	O
"	O
close	O
"	O
will	O
simply	O
be	O
dropped	O
in	O
transit	O
,	O
as	O
though	O
they	O
never	O
existed	O
.	O
</s>
