<s>
IPFilter	B-Protocol
(	O
commonly	O
referred	O
to	O
as	O
ipf	O
)	O
is	O
an	O
open-source	B-Application
software	I-Application
package	O
that	O
provides	O
firewall	B-Application
services	O
and	O
network	B-Protocol
address	I-Protocol
translation	I-Protocol
(	O
NAT	O
)	O
for	O
many	O
Unix-like	B-Operating_System
operating	I-Operating_System
systems	I-Operating_System
.	O
</s>
<s>
IPFilter	B-Protocol
supports	O
both	O
IPv4	O
and	O
IPv6	O
protocols	O
,	O
and	O
is	O
a	O
stateful	O
firewall	B-Application
.	O
</s>
<s>
IPFilter	B-Protocol
is	O
delivered	O
with	O
FreeBSD	B-Operating_System
,	O
NetBSD	B-Device
,	O
Solaris	B-Application
10	I-Application
&	I-Application
11	I-Application
,	O
illumos	B-Application
,	O
OpenIndiana	B-Application
and	O
HP-UX	B-Application
.	O
</s>
<s>
It	O
used	O
to	O
be	O
a	O
part	O
of	O
OpenBSD	B-Operating_System
,	O
but	O
it	O
was	O
removed	O
by	O
Theo	O
de	O
Raadt	O
in	O
May	O
2001	O
due	O
to	O
problems	O
with	O
its	O
license	O
.	O
</s>
<s>
It	O
was	O
subsequently	O
replaced	O
in	O
OpenBSD	B-Operating_System
by	O
PF	B-Protocol
,	O
which	O
was	O
developed	O
by	O
OpenBSD	B-Operating_System
's	O
own	O
developers	O
.	O
</s>
<s>
DragonFly	B-Application
BSD	I-Application
removed	O
its	O
support	O
for	O
IPFilter	B-Protocol
in	O
May	O
2011	O
.	O
</s>
<s>
IPFilter	B-Protocol
can	O
be	O
installed	O
as	O
a	O
runtime-loadable	O
kernel	B-Operating_System
module	O
or	O
directly	O
incorporated	O
into	O
the	O
operating	B-Operating_System
system	I-Operating_System
kernel	I-Operating_System
,	O
depending	O
on	O
the	O
specifics	O
of	O
each	O
kernel	B-Operating_System
and	O
user	O
preferences	O
.	O
</s>
<s>
The	O
software	O
's	O
documentation	O
recommends	O
the	O
module	B-Application
approach	O
,	O
if	O
possible	O
.	O
</s>
