<s>
Internetworking	B-Architecture
is	O
the	O
practice	O
of	O
interconnecting	O
multiple	O
computer	B-Architecture
networks	I-Architecture
,	O
such	O
that	O
any	O
pair	O
of	O
hosts	O
in	O
the	O
connected	O
networks	O
can	O
exchange	O
messages	O
irrespective	O
of	O
their	O
hardware-level	O
networking	O
technology	O
.	O
</s>
<s>
The	O
resulting	O
system	O
of	O
interconnected	O
networks	O
are	O
called	O
an	O
internetwork	B-Architecture
,	O
or	O
simply	O
an	B-Architecture
internet	I-Architecture
.	O
</s>
<s>
The	O
most	O
notable	O
example	O
of	O
internetworking	B-Architecture
is	O
the	O
Internet	O
,	O
a	O
network	O
of	O
networks	O
based	O
on	O
many	O
underlying	O
hardware	O
technologies	O
.	O
</s>
<s>
The	O
Internet	O
is	O
defined	O
by	O
a	O
unified	O
global	B-Protocol
addressing	I-Protocol
system	I-Protocol
,	O
packet	B-Protocol
format	O
,	O
and	O
routing	B-Protocol
methods	O
provided	O
by	O
the	O
Internet	B-Protocol
Protocol	I-Protocol
.	O
</s>
<s>
The	O
term	O
internetworking	B-Architecture
is	O
a	O
combination	O
of	O
the	O
components	O
inter	O
(	O
between	O
)	O
and	O
networking	O
.	O
</s>
<s>
An	O
earlier	O
term	O
for	O
an	O
internetwork	B-Architecture
is	O
catenet	B-Architecture
,	O
a	O
short-form	O
of	O
(	O
con	O
)	O
catenating	O
networks	O
.	O
</s>
<s>
Internetworking	B-Architecture
started	O
as	O
a	O
way	O
to	O
connect	O
disparate	O
types	O
of	O
networking	O
technology	O
,	O
but	O
it	O
became	O
widespread	O
through	O
the	O
developing	O
need	O
to	O
connect	O
two	O
or	O
more	O
local	B-General_Concept
area	I-General_Concept
networks	I-General_Concept
via	O
some	O
sort	O
of	O
wide	O
area	O
network	O
.	O
</s>
<s>
The	O
first	O
international	O
heterogenous	O
resource	B-General_Concept
sharing	I-General_Concept
network	O
was	O
the	O
1973	O
interconnection	O
of	O
the	O
ARPANET	O
with	O
early	O
British	O
academic	O
networks	O
through	O
the	O
computer	O
science	O
department	O
at	O
University	O
College	O
London	O
(	O
UCL	O
)	O
.	O
</s>
<s>
In	O
the	O
ARPANET	O
,	O
the	O
network	O
elements	O
used	O
to	O
connect	O
individual	O
networks	O
were	O
called	O
gateways	B-Application
,	O
but	O
the	O
term	O
has	O
been	O
deprecated	O
in	O
this	O
context	O
,	O
because	O
of	O
possible	O
confusion	O
with	O
functionally	O
different	O
devices	O
.	O
</s>
<s>
By	O
1973-4	O
,	O
researchers	O
in	O
the	O
United	O
States	O
,	O
the	O
United	O
Kingdom	O
and	O
France	O
had	O
worked	O
out	O
an	O
approach	O
to	O
internetworking	B-Architecture
where	O
the	O
differences	O
between	O
network	O
protocols	O
were	O
hidden	O
by	O
using	O
a	O
common	O
internetwork	B-Architecture
protocol	O
,	O
and	O
instead	O
of	O
the	O
network	O
being	O
responsible	O
for	O
reliability	O
,	O
as	O
in	O
the	O
ARPANET	O
,	O
the	O
hosts	O
became	O
responsible	O
,	O
as	O
demonstrated	O
in	O
the	O
CYCLADES	O
network	O
.	O
</s>
<s>
Research	O
at	O
the	O
National	O
Physical	O
Laboratory	O
in	O
the	O
United	O
Kingdom	O
confirmed	O
establishing	O
a	O
common	O
host	B-Device
protocol	O
would	O
be	O
more	O
reliable	O
and	O
efficient	O
.	O
</s>
<s>
In	O
1977	O
,	O
ARPA	O
demonstrated	O
a	O
three-way	O
internetworking	B-Architecture
experiment	O
,	O
which	O
linked	O
a	O
mobile	O
vehicle	O
in	O
PRNET	B-Protocol
with	O
nodes	O
in	O
the	O
ARPANET	O
,	O
and	O
via	O
SATNET	O
,	O
to	O
nodes	O
at	O
UCL	O
.	O
</s>
<s>
The	O
X.25	B-Protocol
protocol	O
,	O
on	O
which	O
public	O
data	B-Architecture
networks	I-Architecture
were	O
based	O
in	O
the	O
1970s	O
and	O
1980s	O
,	O
was	O
supplemented	O
by	O
the	O
X.75	B-Protocol
protocol	O
which	O
enabled	O
internetworking	B-Architecture
.	O
</s>
<s>
Today	O
the	O
interconnecting	O
gateways	B-Application
are	O
called	O
routers	B-Protocol
.	O
</s>
<s>
The	O
definition	O
of	O
an	O
internetwork	B-Architecture
today	O
includes	O
the	O
connection	O
of	O
other	O
types	O
of	O
computer	B-Architecture
networks	I-Architecture
such	O
as	O
personal	B-General_Concept
area	I-General_Concept
networks	I-General_Concept
.	O
</s>
<s>
To	O
build	O
an	O
internetwork	B-Architecture
,	O
the	O
following	O
are	O
needed	O
:	O
A	O
standardized	O
scheme	O
to	O
address	B-Protocol
packets	B-Protocol
to	O
any	O
host	B-Device
on	O
any	O
participating	O
network	O
;	O
a	O
standardized	O
protocol	O
defining	O
format	O
and	O
handling	O
of	O
transmitted	O
packets	B-Protocol
;	O
components	O
interconnecting	O
the	O
participating	O
networks	O
by	O
routing	B-Protocol
packets	B-Protocol
to	O
their	O
destinations	O
based	O
on	O
standardized	O
addresses	O
.	O
</s>
<s>
Another	O
type	O
of	O
interconnection	O
of	O
networks	O
often	O
occurs	O
within	O
enterprises	O
at	O
the	O
link	B-Protocol
layer	I-Protocol
of	O
the	O
networking	O
model	O
,	O
i.e.	O
</s>
<s>
at	O
the	O
hardware-centric	O
layer	O
below	O
the	O
level	O
of	O
the	O
TCP/IP	B-Protocol
logical	O
interfaces	O
.	O
</s>
<s>
Such	O
interconnection	O
is	O
accomplished	O
with	O
network	B-Protocol
bridges	I-Protocol
and	O
network	B-Protocol
switches	I-Protocol
.	O
</s>
<s>
This	O
is	O
sometimes	O
incorrectly	O
termed	O
internetworking	B-Architecture
,	O
but	O
the	O
resulting	O
system	O
is	O
simply	O
a	O
larger	O
,	O
single	O
subnetwork	B-Protocol
,	O
and	O
no	O
internetworking	B-Architecture
protocol	O
,	O
such	O
as	O
Internet	B-Protocol
Protocol	I-Protocol
,	O
is	O
required	O
to	O
traverse	O
these	O
devices	O
.	O
</s>
<s>
However	O
,	O
a	O
single	O
computer	B-Architecture
network	I-Architecture
may	O
be	O
converted	O
into	O
an	O
internetwork	B-Architecture
by	O
dividing	O
the	O
network	O
into	O
segments	O
and	O
logically	O
dividing	O
the	O
segment	O
traffic	O
with	O
routers	B-Protocol
and	O
having	O
an	O
internetworking	B-Architecture
software	O
layer	O
that	O
applications	O
employ	O
.	O
</s>
<s>
The	O
Internet	B-Protocol
Protocol	I-Protocol
is	O
designed	O
to	O
provide	O
an	O
unreliable	B-Protocol
(	O
not	O
guaranteed	O
)	O
packet	B-Protocol
service	I-Protocol
across	O
the	O
network	O
.	O
</s>
<s>
To	O
transfer	O
data	O
reliably	O
,	O
applications	O
must	O
utilize	O
an	O
appropriate	O
transport	B-Architecture
layer	I-Architecture
protocol	O
,	O
such	O
as	O
Transmission	B-Protocol
Control	I-Protocol
Protocol	I-Protocol
(	O
TCP	O
)	O
,	O
which	O
provides	O
a	O
reliable	O
stream	O
.	O
</s>
<s>
Some	O
applications	O
use	O
a	O
simpler	O
,	O
connection-less	O
transport	B-Architecture
protocol	I-Architecture
,	O
User	B-Protocol
Datagram	I-Protocol
Protocol	I-Protocol
(	O
UDP	O
)	O
,	O
for	O
tasks	O
which	O
do	O
not	O
require	O
reliable	B-Protocol
delivery	I-Protocol
of	O
data	O
or	O
that	O
require	O
real-time	O
service	O
,	O
such	O
as	O
video	B-Application
streaming	I-Application
or	O
voice	O
chat	O
.	O
</s>
<s>
Catenet	B-Architecture
is	O
an	O
obsolete	O
term	O
for	O
a	O
system	O
of	O
packet-switched	B-Protocol
communication	O
networks	O
interconnected	O
via	O
gateways	B-Application
.	O
</s>
<s>
The	O
term	O
was	O
coined	O
by	O
Louis	O
Pouzin	O
in	O
October	O
1973	O
in	O
a	O
note	O
circulated	O
to	O
the	O
International	B-Protocol
Networking	I-Protocol
Working	I-Protocol
Group	I-Protocol
,	O
later	O
published	O
in	O
a	O
1974	O
paper	O
"	O
A	O
Proposal	O
for	O
Interconnecting	O
Packet	B-Protocol
Switching	I-Protocol
Networks	O
"	O
.	O
</s>
<s>
Pouzin	O
was	O
a	O
pioneer	O
in	O
packet-switching	B-Protocol
technology	O
and	O
founder	O
of	O
the	O
CYCLADES	O
network	O
,	O
at	O
a	O
time	O
when	O
network	O
meant	O
what	O
is	O
now	O
called	O
a	O
local	B-General_Concept
area	I-General_Concept
network	I-General_Concept
.	O
</s>
<s>
Catenet	B-Architecture
was	O
the	O
concept	O
of	O
linking	O
these	O
networks	O
into	O
a	O
network	O
of	O
networks	O
with	O
specifications	O
for	O
compatibility	O
of	O
addressing	O
and	O
routing	B-Protocol
.	O
</s>
<s>
The	O
term	O
catenet	B-Architecture
was	O
gradually	O
displaced	O
by	O
the	O
short-form	O
of	O
the	O
term	O
internetwork	B-Architecture
,	O
internet	O
(	O
lower-case	O
i	O
)	O
,	O
when	O
the	O
Internet	B-Protocol
Protocol	I-Protocol
replaced	O
earlier	O
protocols	O
on	O
the	O
ARPANET	O
.	O
</s>
<s>
Two	O
architectural	O
models	O
are	O
commonly	O
used	O
to	O
describe	O
the	O
protocols	O
and	O
methods	O
used	O
in	O
internetworking	B-Architecture
.	O
</s>
<s>
Internetworking	B-Architecture
is	O
implemented	O
in	O
the	O
Network	B-Application
Layer	I-Application
(	O
Layer	B-Application
3	I-Application
)	O
of	O
the	O
model	O
.	O
</s>
<s>
The	O
Internet	B-Protocol
Protocol	I-Protocol
Suite	I-Protocol
,	O
also	O
known	O
as	O
the	O
TCP/IP	B-Protocol
model	I-Protocol
,	O
was	O
not	O
designed	O
to	O
conform	O
to	O
the	O
OSI	O
model	O
and	O
does	O
not	O
refer	O
to	O
it	O
in	O
any	O
of	O
the	O
normative	O
specifications	O
in	O
Request	O
for	O
Comments	O
and	O
Internet	O
standards	O
.	O
</s>
<s>
It	O
assumes	O
the	O
availability	O
of	O
any	O
suitable	O
hardware	O
infrastructure	O
,	O
without	O
discussing	O
hardware-specific	O
low-level	O
interfaces	O
,	O
and	O
that	O
a	O
host	B-Device
has	O
access	O
to	O
this	O
local	B-General_Concept
network	I-General_Concept
to	O
which	O
it	O
is	O
connected	O
via	O
a	O
link	B-Protocol
layer	I-Protocol
interface	O
.	O
</s>
<s>
For	O
a	O
period	O
in	O
the	O
late	O
1980s	O
and	O
early	O
1990s	O
,	O
the	O
network	O
engineering	O
community	O
was	O
polarized	O
over	O
the	O
implementation	O
of	O
competing	O
protocol	O
suites	O
,	O
commonly	O
known	O
as	O
the	O
Protocol	B-Protocol
Wars	I-Protocol
.	O
</s>
<s>
It	O
was	O
unclear	O
which	O
of	O
the	O
OSI	O
model	O
and	O
the	O
Internet	B-Protocol
protocol	I-Protocol
suite	I-Protocol
would	O
result	O
in	O
the	O
best	O
and	O
most	O
robust	O
computer	B-Architecture
networks	I-Architecture
.	O
</s>
