<s>
Longest	B-Protocol
prefix	I-Protocol
match	I-Protocol
(	O
also	O
called	O
Maximum	O
prefix	O
length	O
match	O
)	O
refers	O
to	O
an	O
algorithm	O
used	O
by	O
routers	B-Protocol
in	O
Internet	B-Protocol
Protocol	I-Protocol
(	O
IP	O
)	O
networking	O
to	O
select	O
an	O
entry	O
from	O
a	O
routing	O
table	O
.	O
</s>
<s>
Because	O
each	O
entry	O
in	O
a	O
forwarding	B-Protocol
table	I-Protocol
may	O
specify	O
a	O
sub-network	B-Protocol
,	O
one	O
destination	O
address	O
may	O
match	O
more	O
than	O
one	O
forwarding	B-Protocol
table	I-Protocol
entry	O
.	O
</s>
<s>
The	O
most	O
specific	O
of	O
the	O
matching	O
table	O
entries	O
the	O
one	O
with	O
the	O
longest	O
subnet	B-Protocol
mask	I-Protocol
is	O
called	O
the	O
longest	B-Protocol
prefix	I-Protocol
match	I-Protocol
.	O
</s>
<s>
For	O
example	O
,	O
consider	O
this	O
IPv4	B-Protocol
forwarding	B-Protocol
table	I-Protocol
(	O
CIDR	B-Protocol
notation	I-Protocol
is	O
used	O
)	O
:	O
</s>
<s>
When	O
the	O
address	O
192.168.20.19	O
needs	O
to	O
be	O
looked	O
up	O
,	O
both	O
entries	O
in	O
the	O
forwarding	B-Protocol
table	I-Protocol
"	O
match	O
"	O
.	O
</s>
<s>
In	O
this	O
case	O
,	O
the	O
longest	O
prefix	O
of	O
the	O
candidate	O
routes	O
is	O
192.168.20.16/28	O
,	O
since	O
its	O
subnet	B-Protocol
mask	I-Protocol
(	O
/28	O
)	O
is	O
longer	O
than	O
the	O
other	O
entry	O
's	O
mask	O
(	O
/16	O
)	O
,	O
making	O
the	O
route	O
more	O
specific	O
.	O
</s>
<s>
Forwarding	B-Protocol
tables	I-Protocol
often	O
contain	O
a	O
default	B-Protocol
route	I-Protocol
,	O
which	O
has	O
the	O
shortest	O
possible	O
prefix	O
match	O
,	O
to	O
fall	O
back	O
on	O
in	O
case	O
matches	O
with	O
all	O
other	O
entries	O
fail	O
.	O
</s>
