<s>
A	O
wildcard	B-Protocol
mask	I-Protocol
is	O
a	O
mask	O
of	O
bits	O
that	O
indicates	O
which	O
parts	O
of	O
an	O
IP	B-Protocol
address	I-Protocol
are	O
available	O
for	O
examination	O
.	O
</s>
<s>
In	O
the	O
Cisco	B-Protocol
IOS	I-Protocol
,	O
they	O
are	O
used	O
in	O
several	O
places	O
,	O
for	O
example	O
:	O
</s>
<s>
To	O
indicate	O
the	O
size	O
of	O
a	O
network	O
or	O
subnet	B-Protocol
for	O
some	O
routing	O
protocols	O
,	O
such	O
as	O
OSPF	B-Protocol
.	O
</s>
<s>
To	O
indicate	O
what	O
IP	B-Protocol
addresses	I-Protocol
should	O
be	O
permitted	O
or	O
denied	O
in	O
access	O
control	O
lists	O
(	O
ACLs	O
)	O
.	O
</s>
<s>
A	O
wildcard	B-Protocol
mask	I-Protocol
can	O
be	O
thought	O
of	O
as	O
an	O
inverted	O
subnet	B-Protocol
mask	I-Protocol
.	O
</s>
<s>
For	O
example	O
,	O
a	O
subnet	B-Protocol
mask	I-Protocol
of	O
255.255.255.0	O
(	O
binary	O
equivalent	O
=	O
11111111.11111111.11111111.00000000	O
)	O
inverts	O
to	O
a	O
wildcard	B-Protocol
mask	I-Protocol
of	O
0.0.0.255	O
(	O
binary	O
equivalent	O
=	O
00000000.00000000.00000000.11111111	O
)	O
.	O
</s>
<s>
A	O
wild	B-Protocol
card	I-Protocol
mask	I-Protocol
is	O
a	O
matching	O
rule	O
.	O
</s>
<s>
The	O
rule	O
for	O
a	O
wildcard	B-Protocol
mask	I-Protocol
is	O
:	O
</s>
<s>
For	O
example	O
,	O
a	O
wildcard	B-Protocol
mask	I-Protocol
of	O
0.0.0.254	O
(	O
binary	O
equivalent	O
=	O
00000000.00000000.00000000.11111110	O
)	O
applied	O
to	O
IP	B-Protocol
address	I-Protocol
10.10.10.2	O
(	O
00001010.00001010.00001010.00000010	O
)	O
will	O
match	O
even-numbered	O
IP	B-Protocol
addresses	I-Protocol
10.10.10.0	O
,	O
10.10.10.2	O
,	O
10.10.10.4	O
,	O
10.10.10.6	O
etc	O
.	O
</s>
<s>
Same	O
mask	O
applied	O
to	O
10.10.10.1	O
(	O
00001010.00001010.00001010.00000001	O
)	O
will	O
match	O
odd-numbered	O
IP	B-Protocol
addresses	I-Protocol
10.10.10.1	O
,	O
10.10.10.3	O
,	O
10.10.10.5	O
etc	O
.	O
</s>
<s>
A	O
network	O
and	O
wildcard	B-Protocol
mask	I-Protocol
combination	O
of	O
1.1.1.1	O
0.0.0.0	O
would	O
match	O
an	O
interface	O
configured	O
exactly	O
with	O
1.1.1.1	O
only	O
,	O
and	O
nothing	O
else	O
.	O
</s>
<s>
Wildcard	B-Protocol
masks	I-Protocol
are	O
used	O
in	O
situations	O
where	O
subnet	B-Protocol
masks	I-Protocol
may	O
not	O
apply	O
.	O
</s>
<s>
For	O
example	O
,	O
when	O
two	O
affected	O
hosts	O
fall	O
in	O
different	O
subnets	B-Protocol
,	O
the	O
use	O
of	O
a	O
wildcard	B-Protocol
mask	I-Protocol
will	O
group	O
them	O
together	O
.	O
</s>
