<s>
In	O
computer	B-Architecture
networking	I-Architecture
,	O
a	O
routing	B-Protocol
table	I-Protocol
,	O
or	O
routing	B-Protocol
information	I-Protocol
base	I-Protocol
(	O
RIB	O
)	O
,	O
is	O
a	O
data	B-Application
table	I-Application
stored	O
in	O
a	O
router	B-Protocol
or	O
a	O
network	B-Device
host	I-Device
that	O
lists	O
the	O
routes	O
to	O
particular	O
network	O
destinations	O
,	O
and	O
in	O
some	O
cases	O
,	O
metrics	O
(	O
distances	O
)	O
associated	O
with	O
those	O
routes	O
.	O
</s>
<s>
The	O
routing	B-Protocol
table	I-Protocol
contains	O
information	O
about	O
the	O
topology	B-Architecture
of	I-Architecture
the	I-Architecture
network	I-Architecture
immediately	O
around	O
it	O
.	O
</s>
<s>
The	O
construction	O
of	O
routing	B-Protocol
tables	I-Protocol
is	O
the	O
primary	O
goal	O
of	O
routing	B-Protocol
protocols	I-Protocol
.	O
</s>
<s>
Static	B-Protocol
routes	I-Protocol
are	O
entries	O
that	O
are	O
fixed	O
,	O
rather	O
than	O
resulting	O
from	O
routing	B-Protocol
protocols	I-Protocol
and	O
network	B-Architecture
topology	I-Architecture
discovery	O
procedures	O
.	O
</s>
<s>
A	O
routing	B-Protocol
table	I-Protocol
is	O
analogous	O
to	O
a	O
distribution	O
map	O
in	O
package	O
delivery	O
.	O
</s>
<s>
Whenever	O
a	O
node	B-Protocol
needs	O
to	O
send	O
data	O
to	O
another	O
node	B-Protocol
on	O
a	O
network	O
,	O
it	O
must	O
first	O
know	O
where	O
to	O
send	O
it	O
.	O
</s>
<s>
If	O
the	O
node	B-Protocol
cannot	O
directly	O
connect	O
to	O
the	O
destination	O
node	B-Protocol
,	O
it	O
has	O
to	O
send	O
it	O
via	O
other	O
nodes	O
along	O
a	O
route	O
to	O
the	O
destination	O
node	B-Protocol
.	O
</s>
<s>
Each	O
node	B-Protocol
needs	O
to	O
keep	O
track	O
of	O
which	O
way	O
to	O
deliver	O
various	O
packages	O
of	O
data	O
,	O
and	O
for	O
this	O
it	O
uses	O
a	O
routing	B-Protocol
table	I-Protocol
.	O
</s>
<s>
A	O
routing	B-Protocol
table	I-Protocol
is	O
a	O
database	O
that	O
keeps	O
track	O
of	O
paths	O
,	O
like	O
a	O
map	O
,	O
and	O
uses	O
these	O
to	O
determine	O
which	O
way	O
to	O
forward	O
traffic	O
.	O
</s>
<s>
A	O
routing	B-Protocol
table	I-Protocol
is	O
a	O
data	O
file	O
in	O
RAM	O
that	O
is	O
used	O
to	O
store	O
route	O
information	O
about	O
directly	O
connected	O
and	O
remote	O
networks	O
.	O
</s>
<s>
Nodes	O
can	O
also	O
share	O
the	O
contents	O
of	O
their	O
routing	B-Protocol
table	I-Protocol
with	O
other	O
nodes	O
.	O
</s>
<s>
The	O
primary	O
function	O
of	O
a	O
router	B-Protocol
is	O
to	O
forward	O
a	O
packet	B-Protocol
toward	O
its	O
destination	O
network	O
,	O
which	O
is	O
the	O
destination	O
IP	O
address	O
of	O
the	O
packet	B-Protocol
.	O
</s>
<s>
To	O
do	O
this	O
,	O
a	O
router	B-Protocol
needs	O
to	O
search	O
the	O
routing	B-Protocol
information	I-Protocol
stored	O
in	O
its	O
routing	B-Protocol
table	I-Protocol
.	O
</s>
<s>
The	O
routing	B-Protocol
table	I-Protocol
contains	O
network/next	O
hop	B-Protocol
associations	O
.	O
</s>
<s>
These	O
associations	O
tell	O
a	O
router	B-Protocol
that	O
a	O
particular	O
destination	O
can	O
be	O
optimally	O
reached	O
by	O
sending	O
the	O
packet	B-Protocol
to	O
a	O
specific	O
router	B-Protocol
that	O
represents	O
the	O
next	O
hop	B-Protocol
on	O
the	O
way	O
to	O
the	O
final	O
destination	O
.	O
</s>
<s>
The	O
next	O
hop	B-Protocol
association	O
can	O
also	O
be	O
the	O
outgoing	O
or	O
exit	O
interface	O
to	O
the	O
final	O
destination	O
.	O
</s>
<s>
With	O
hop-by-hop	O
routing	B-Protocol
,	O
each	O
routing	B-Protocol
table	I-Protocol
lists	O
,	O
for	O
all	O
reachable	O
destinations	O
,	O
the	O
address	O
of	O
the	O
next	O
device	O
along	O
the	O
path	O
to	O
that	O
destination	O
:	O
the	O
next	O
hop	B-Protocol
.	O
</s>
<s>
Assuming	O
that	O
the	O
routing	B-Protocol
tables	I-Protocol
are	O
consistent	O
,	O
the	O
simple	O
algorithm	O
of	O
relaying	O
packets	B-Protocol
to	O
their	O
destination	O
's	O
next	O
hop	B-Protocol
thus	O
suffices	O
to	O
deliver	O
data	O
anywhere	O
in	O
a	O
network	O
.	O
</s>
<s>
Hop-by-hop	O
is	O
the	O
fundamental	O
characteristic	O
of	O
the	O
IP	O
Internet	B-Protocol
layer	I-Protocol
and	O
the	O
OSI	O
Network	B-Application
Layer	I-Application
.	O
</s>
<s>
When	O
a	O
router	B-Protocol
interface	O
is	O
configured	O
with	O
an	O
IP	O
address	O
and	O
subnet	O
mask	O
,	O
the	O
interface	O
becomes	O
a	O
host	O
on	O
that	O
attached	O
network	O
.	O
</s>
<s>
A	O
directly	O
connected	O
network	O
is	O
a	O
network	O
that	O
is	O
directly	O
attached	O
to	O
one	O
of	O
the	O
router	B-Protocol
interfaces	O
.	O
</s>
<s>
The	O
network	O
address	O
and	O
subnet	O
mask	O
of	O
the	O
interface	O
,	O
along	O
with	O
the	O
interface	O
type	O
and	O
number	O
,	O
are	O
entered	O
into	O
the	O
routing	B-Protocol
table	I-Protocol
as	O
a	O
directly	O
connected	O
network	O
.	O
</s>
<s>
A	O
remote	O
network	O
is	O
a	O
network	O
that	O
can	O
only	O
be	O
reached	O
by	O
sending	O
the	O
packet	B-Protocol
to	O
another	O
router	B-Protocol
.	O
</s>
<s>
Routing	B-Protocol
table	I-Protocol
entries	O
to	O
remote	O
networks	O
may	O
be	O
either	O
dynamic	O
or	O
static	O
.	O
</s>
<s>
Dynamic	O
routes	O
are	O
routes	O
to	O
remote	O
networks	O
that	O
were	O
learned	O
automatically	O
by	O
the	O
router	B-Protocol
through	O
a	O
dynamic	O
routing	B-Protocol
protocol	I-Protocol
.	O
</s>
<s>
Static	B-Protocol
routes	I-Protocol
are	O
routes	O
that	O
a	O
network	O
administrator	O
manually	O
configured	O
.	O
</s>
<s>
Routing	B-Protocol
tables	I-Protocol
are	O
also	O
a	O
key	O
aspect	O
of	O
certain	O
security	O
operations	O
,	O
such	O
as	O
unicast	O
reverse	O
path	O
forwarding	O
(	O
uRPF	O
)	O
.	O
</s>
<s>
In	O
this	O
technique	O
,	O
which	O
has	O
several	O
variants	O
,	O
the	O
router	B-Protocol
also	O
looks	O
up	O
,	O
in	O
the	O
routing	B-Protocol
table	I-Protocol
,	O
the	O
source	O
address	O
of	O
the	O
packet	B-Protocol
.	O
</s>
<s>
If	O
there	O
exists	O
no	O
route	O
back	O
to	O
the	O
source	O
address	O
,	O
the	O
packet	B-Protocol
is	O
assumed	O
to	O
be	O
malformed	O
or	O
involved	O
in	O
a	O
network	O
attack	O
and	O
is	O
dropped	O
.	O
</s>
<s>
The	O
need	O
to	O
record	O
routes	O
to	O
large	O
numbers	O
of	O
devices	O
using	O
limited	O
storage	O
space	O
represents	O
a	O
major	O
challenge	O
in	O
routing	B-Protocol
table	I-Protocol
construction	O
.	O
</s>
<s>
In	O
the	O
Internet	O
,	O
the	O
currently	O
dominant	O
address	O
aggregation	O
technology	O
is	O
a	O
bitwise	O
prefix	O
matching	O
scheme	O
called	O
Classless	B-Protocol
Inter-Domain	I-Protocol
Routing	I-Protocol
(	O
CIDR	B-Protocol
)	O
.	O
</s>
<s>
Supernetworks	B-Protocol
can	O
also	O
be	O
used	O
to	O
help	O
control	O
routing	B-Protocol
table	I-Protocol
size	O
.	O
</s>
<s>
The	O
routing	B-Protocol
table	I-Protocol
consists	O
of	O
at	O
least	O
three	O
information	O
fields	O
:	O
</s>
<s>
metric	O
:	O
The	O
routing	B-Protocol
metric	O
of	O
the	O
path	O
through	O
which	O
the	O
packet	B-Protocol
is	O
to	O
be	O
sent	O
.	O
</s>
<s>
Shown	O
below	O
is	O
an	O
example	O
of	O
what	O
the	O
table	O
above	O
could	O
look	O
like	O
on	O
a	O
computer	O
connected	O
to	O
the	O
internet	O
via	O
a	O
home	B-Application
router	I-Application
:	O
</s>
<s>
The	O
Gateway	O
column	O
contains	O
the	O
same	O
information	O
as	O
the	O
Next	O
hop	B-Protocol
,	O
i.e.	O
</s>
<s>
In	O
this	O
example	O
,	O
gateway	O
192.168.0.1	O
(	O
the	O
internet	B-Protocol
router	I-Protocol
)	O
can	O
be	O
reached	O
through	O
the	O
local	O
network	O
card	O
with	O
address	O
192.168.0.100	O
.	O
</s>
<s>
Routing	B-Protocol
tables	I-Protocol
are	O
generally	O
not	O
used	O
directly	O
for	O
packet	B-Protocol
forwarding	I-Protocol
in	O
modern	O
router	B-Protocol
architectures	O
;	O
instead	O
,	O
they	O
are	O
used	O
to	O
generate	O
the	O
information	O
for	O
a	O
simpler	O
forwarding	B-Protocol
table	I-Protocol
.	O
</s>
<s>
This	O
forwarding	B-Protocol
table	I-Protocol
contains	O
only	O
the	O
routes	O
which	O
are	O
chosen	O
by	O
the	O
routing	B-Protocol
algorithm	I-Protocol
as	O
preferred	O
routes	O
for	O
packet	B-Protocol
forwarding	I-Protocol
.	O
</s>
<s>
It	O
is	O
often	O
in	O
a	O
compressed	O
or	O
pre-compiled	O
format	O
that	O
is	O
optimized	O
for	O
hardware	O
storage	O
and	O
lookup	B-Data_Structure
.	O
</s>
<s>
This	O
router	B-Protocol
architecture	O
separates	O
the	O
control	B-Protocol
plane	I-Protocol
function	O
of	O
the	O
routing	B-Protocol
table	I-Protocol
from	O
the	O
forwarding	B-Protocol
plane	I-Protocol
function	O
of	O
the	O
forwarding	B-Protocol
table	I-Protocol
.	O
</s>
