<s>
The	O
Internet	B-Protocol
Protocol	I-Protocol
(	O
IP	O
)	O
is	O
the	O
network	B-Application
layer	I-Application
communications	O
protocol	O
in	O
the	O
Internet	B-Protocol
protocol	I-Protocol
suite	I-Protocol
for	O
relaying	O
datagrams	B-Protocol
across	O
network	O
boundaries	O
.	O
</s>
<s>
Its	O
routing	B-Protocol
function	O
enables	O
internetworking	B-Architecture
,	O
and	O
essentially	O
establishes	O
the	O
Internet	O
.	O
</s>
<s>
IP	O
has	O
the	O
task	O
of	O
delivering	O
packets	B-Protocol
from	O
the	O
source	O
host	B-Device
to	O
the	O
destination	O
host	B-Device
solely	O
based	O
on	O
the	O
IP	B-Protocol
addresses	I-Protocol
in	O
the	O
packet	B-General_Concept
headers	I-General_Concept
.	O
</s>
<s>
For	O
this	O
purpose	O
,	O
IP	O
defines	O
packet	B-Protocol
structures	O
that	O
encapsulate	B-Protocol
the	O
data	O
to	O
be	O
delivered	O
.	O
</s>
<s>
It	O
also	O
defines	O
addressing	O
methods	O
that	O
are	O
used	O
to	O
label	O
the	O
datagram	B-Protocol
with	O
source	O
and	O
destination	O
information	O
.	O
</s>
<s>
IP	O
was	O
the	O
connectionless	B-Protocol
datagram	B-Protocol
service	I-Protocol
in	O
the	O
original	O
Transmission	B-Protocol
Control	I-Protocol
Program	I-Protocol
introduced	O
by	O
Vint	O
Cerf	O
and	O
Bob	O
Kahn	O
in	O
1974	O
,	O
which	O
was	O
complemented	O
by	O
a	O
connection-oriented	B-Protocol
service	I-Protocol
that	O
became	O
the	O
basis	O
for	O
the	O
Transmission	B-Protocol
Control	I-Protocol
Protocol	I-Protocol
(	O
TCP	O
)	O
.	O
</s>
<s>
The	O
Internet	B-Protocol
protocol	I-Protocol
suite	I-Protocol
is	O
therefore	O
often	O
referred	O
to	O
as	O
TCP/IP	B-Protocol
.	O
</s>
<s>
The	O
first	O
major	O
version	O
of	O
IP	O
,	O
Internet	B-Protocol
Protocol	I-Protocol
Version	I-Protocol
4	I-Protocol
(	O
IPv4	B-Protocol
)	O
,	O
is	O
the	O
dominant	O
protocol	O
of	O
the	O
Internet	O
.	O
</s>
<s>
Its	O
successor	O
is	O
Internet	B-Protocol
Protocol	I-Protocol
Version	I-Protocol
6	I-Protocol
(	O
IPv6	B-Protocol
)	O
,	O
which	O
has	O
been	O
in	O
increasing	O
deployment	B-Architecture
on	O
the	O
public	O
Internet	O
since	O
c	O
.	O
2006	O
.	O
</s>
<s>
The	O
Internet	B-Protocol
Protocol	I-Protocol
is	O
responsible	O
for	O
addressing	O
host	B-Protocol
interfaces	I-Protocol
,	O
encapsulating	O
data	O
into	O
datagrams	B-Protocol
(	O
including	O
fragmentation	B-Protocol
and	I-Protocol
reassembly	I-Protocol
)	O
and	O
routing	B-Protocol
datagrams	B-Protocol
from	O
a	O
source	O
host	B-Protocol
interface	I-Protocol
to	O
a	O
destination	O
host	B-Protocol
interface	I-Protocol
across	O
one	O
or	O
more	O
IP	B-Protocol
networks	I-Protocol
.	O
</s>
<s>
For	O
these	O
purposes	O
,	O
the	O
Internet	B-Protocol
Protocol	I-Protocol
defines	O
the	O
format	O
of	O
packets	B-Protocol
and	O
provides	O
an	O
addressing	O
system	O
.	O
</s>
<s>
Each	O
datagram	B-Protocol
has	O
two	O
components	O
:	O
a	O
header	B-General_Concept
and	O
a	O
payload	O
.	O
</s>
<s>
The	O
IP	B-Protocol
header	I-Protocol
includes	O
source	O
IP	B-Protocol
address	I-Protocol
,	O
destination	O
IP	B-Protocol
address	I-Protocol
,	O
and	O
other	O
metadata	O
needed	O
to	O
route	O
and	O
deliver	O
the	O
datagram	B-Protocol
.	O
</s>
<s>
This	O
method	O
of	O
nesting	O
the	O
data	O
payload	O
in	O
a	O
packet	B-Protocol
with	O
a	O
header	B-General_Concept
is	O
called	O
encapsulation	B-Protocol
.	O
</s>
<s>
IP	B-Protocol
addressing	I-Protocol
entails	O
the	O
assignment	O
of	O
IP	B-Protocol
addresses	I-Protocol
and	O
associated	O
parameters	O
to	O
host	B-Protocol
interfaces	I-Protocol
.	O
</s>
<s>
The	O
address	O
space	O
is	O
divided	O
into	O
subnetworks	B-Protocol
,	O
involving	O
the	O
designation	O
of	O
network	O
prefixes	O
.	O
</s>
<s>
IP	B-Protocol
routing	I-Protocol
is	O
performed	O
by	O
all	O
hosts	O
,	O
as	O
well	O
as	O
routers	B-Protocol
,	O
whose	O
main	O
function	O
is	O
to	O
transport	O
packets	B-Protocol
across	O
network	O
boundaries	O
.	O
</s>
<s>
Routers	B-Protocol
communicate	O
with	O
one	O
another	O
via	O
specially	O
designed	O
routing	B-Protocol
protocols	I-Protocol
,	O
either	O
interior	B-Protocol
gateway	I-Protocol
protocols	I-Protocol
or	O
exterior	B-Protocol
gateway	I-Protocol
protocols	I-Protocol
,	O
as	O
needed	O
for	O
the	O
topology	O
of	O
the	O
network	O
.	O
</s>
<s>
In	O
May	O
1974	O
,	O
the	O
Institute	O
of	O
Electrical	O
and	O
Electronics	O
Engineers	O
(	O
IEEE	O
)	O
published	O
a	O
paper	O
entitled	O
"	O
A	O
Protocol	O
for	O
Packet	B-Protocol
Network	I-Protocol
Intercommunication	O
"	O
.	O
</s>
<s>
The	O
paper	O
's	O
authors	O
,	O
Vint	O
Cerf	O
and	O
Bob	O
Kahn	O
,	O
described	O
an	O
internetworking	B-Architecture
protocol	O
for	O
sharing	O
resources	O
using	O
packet	B-Protocol
switching	I-Protocol
among	O
network	B-Protocol
nodes	I-Protocol
.	O
</s>
<s>
A	O
central	O
control	O
component	O
of	O
this	O
model	O
was	O
the	O
"	O
Transmission	B-Protocol
Control	I-Protocol
Program	I-Protocol
"	O
that	O
incorporated	O
both	O
connection-oriented	B-Protocol
links	I-Protocol
and	O
datagram	B-Protocol
services	I-Protocol
between	O
hosts	O
.	O
</s>
<s>
The	O
monolithic	O
Transmission	B-Protocol
Control	I-Protocol
Program	I-Protocol
was	O
later	O
divided	O
into	O
a	O
modular	O
architecture	O
consisting	O
of	O
the	O
Transmission	B-Protocol
Control	I-Protocol
Protocol	I-Protocol
and	O
User	B-Protocol
Datagram	I-Protocol
Protocol	I-Protocol
at	O
the	O
transport	B-Architecture
layer	I-Architecture
and	O
the	O
Internet	B-Protocol
Protocol	I-Protocol
at	O
the	O
internet	B-Protocol
layer	I-Protocol
.	O
</s>
<s>
The	O
model	O
became	O
known	O
as	O
the	O
Department	O
of	O
Defense	O
(	O
DoD	O
)	O
Internet	B-Protocol
Model	I-Protocol
and	O
Internet	B-Protocol
protocol	I-Protocol
suite	I-Protocol
,	O
and	O
informally	O
as	O
TCP/IP	B-Protocol
.	O
</s>
<s>
The	O
following	O
Internet	O
Experiment	O
Note	O
(	O
IEN	O
)	O
documents	O
describe	O
version	O
3	O
of	O
the	O
Internet	B-Protocol
Protocol	I-Protocol
,	O
prior	O
to	O
the	O
modern	O
version	O
of	O
IPv4	B-Protocol
:	O
</s>
<s>
(	O
Comments	O
on	O
Internet	B-Protocol
Protocol	I-Protocol
and	O
TCP	O
)	O
,	O
dated	O
August	O
1977	O
describes	O
the	O
need	O
to	O
separate	O
the	O
TCP	O
and	O
Internet	B-Protocol
Protocol	I-Protocol
functionalities	O
(	O
which	O
were	O
previously	O
combined	O
)	O
.	O
</s>
<s>
It	O
proposes	O
the	O
first	O
version	O
of	O
the	O
IP	B-Protocol
header	I-Protocol
,	O
using	O
0	O
for	O
the	O
version	O
field	O
.	O
</s>
<s>
(	O
A	O
Proposed	O
New	O
Internet	O
Header	B-General_Concept
Format	O
)	O
,	O
dated	O
February	O
1978	O
describes	O
a	O
version	O
of	O
the	O
IP	B-Protocol
header	I-Protocol
that	O
uses	O
a	O
1-bit	O
version	O
field	O
.	O
</s>
<s>
(	O
Draft	O
Internetwork	B-Architecture
Protocol	O
Description	O
Version	O
2	O
)	O
,	O
dated	O
February	O
1978	O
describes	O
IPv2	B-Protocol
.	O
</s>
<s>
(	O
Internetwork	B-Architecture
Protocol	O
Specification	O
Version	O
4	O
)	O
,	O
dated	O
June	O
1978	O
describes	O
the	O
first	O
protocol	O
to	O
be	O
called	O
IPv4	B-Protocol
.	O
</s>
<s>
The	O
IP	B-Protocol
header	I-Protocol
is	O
different	O
from	O
the	O
modern	O
IPv4	B-Protocol
header	B-General_Concept
.	O
</s>
<s>
(	O
Latest	O
Header	B-General_Concept
Formats	O
)	O
,	O
dated	O
June	O
1978	O
describes	O
another	O
version	O
of	O
IPv4	B-Protocol
,	O
also	O
with	O
a	O
header	B-General_Concept
different	O
from	O
the	O
modern	O
IPv4	B-Protocol
header	B-General_Concept
.	O
</s>
<s>
(	O
Internetwork	B-Architecture
Protocol	O
Specification	O
Version	O
4	O
)	O
,	O
dated	O
September	O
1978	O
is	O
the	O
first	O
description	O
of	O
IPv4	B-Protocol
using	O
the	O
header	B-General_Concept
that	O
would	O
be	O
standardized	O
in	O
.	O
</s>
<s>
The	O
dominant	O
internetworking	B-Architecture
protocol	O
in	O
the	O
Internet	B-Protocol
Layer	I-Protocol
in	O
use	O
is	O
IPv4	B-Protocol
;	O
the	O
number	O
4	O
identifies	O
the	O
protocol	O
version	O
,	O
carried	O
in	O
every	O
IP	O
datagram	B-Protocol
.	O
</s>
<s>
IPv4	B-Protocol
is	O
described	O
in	O
(	O
1981	O
)	O
.	O
</s>
<s>
Versions	O
2	O
and	O
3	O
,	O
and	O
a	O
draft	O
of	O
version	O
4	O
,	O
allowed	O
an	O
address	O
length	O
of	O
up	O
to	O
128	O
bits	O
,	O
but	O
this	O
was	O
mistakenly	O
reduced	O
to	O
32	O
bits	O
in	O
the	O
final	O
version	O
of	O
IPv4	B-Protocol
.	O
</s>
<s>
Version	O
number	O
5	O
was	O
used	O
by	O
the	O
Internet	B-Protocol
Stream	I-Protocol
Protocol	I-Protocol
,	O
an	O
experimental	O
streaming	O
protocol	O
that	O
was	O
not	O
adopted	O
.	O
</s>
<s>
The	O
successor	O
to	O
IPv4	B-Protocol
is	O
IPv6	B-Protocol
.	O
</s>
<s>
IPv6	B-Protocol
was	O
a	O
result	O
of	O
several	O
years	O
of	O
experimentation	O
and	O
dialog	O
during	O
which	O
various	O
protocol	O
models	O
were	O
proposed	O
,	O
such	O
as	O
TP/IX	O
(	O
)	O
,	O
PIP	O
(	O
)	O
and	O
TUBA	O
(	O
TCP	O
and	O
UDP	B-Protocol
with	O
Bigger	O
Addresses	O
,	O
)	O
.	O
</s>
<s>
While	O
IPv4	B-Protocol
uses	O
32	O
bits	O
for	O
addressing	O
,	O
yielding	O
c	O
.	O
4.3	O
billion	O
(	O
)	O
addresses	O
,	O
IPv6	B-Protocol
uses	O
128-bit	O
addresses	O
providing	O
c	O
.	O
addresses	O
.	O
</s>
<s>
Although	O
adoption	O
of	O
IPv6	B-Protocol
has	O
been	O
slow	O
,	O
,	O
most	O
countries	O
in	O
the	O
world	O
show	O
significant	O
adoption	O
of	O
IPv6	B-Protocol
,	O
with	O
over	O
35%	O
of	O
Google	O
's	O
traffic	O
being	O
carried	O
over	O
IPv6	B-Protocol
connections	O
.	O
</s>
<s>
The	O
assignment	O
of	O
the	O
new	O
protocol	O
as	O
IPv6	B-Protocol
was	O
uncertain	O
until	O
due	O
diligence	O
assured	O
that	O
IPv6	B-Protocol
had	O
not	O
been	O
used	O
previously	O
.	O
</s>
<s>
Other	O
Internet	B-Protocol
Layer	I-Protocol
protocols	O
have	O
been	O
assigned	O
version	O
numbers	O
,	O
such	O
as	O
7	O
(	O
IP/TX	O
)	O
,	O
8	O
and	O
9	O
(	O
historic	O
)	O
.	O
</s>
<s>
The	O
design	O
of	O
the	O
Internet	B-Protocol
protocol	I-Protocol
suite	I-Protocol
adheres	O
to	O
the	O
end-to-end	B-Application
principle	I-Application
,	O
a	O
concept	O
adapted	O
from	O
the	O
CYCLADES	O
project	O
.	O
</s>
<s>
Under	O
the	O
end-to-end	B-Application
principle	I-Application
,	O
the	O
network	O
infrastructure	O
is	O
considered	O
inherently	O
unreliable	B-Protocol
at	O
any	O
single	O
network	O
element	O
or	O
transmission	O
medium	O
and	O
is	O
dynamic	O
in	O
terms	O
of	O
the	O
availability	O
of	O
links	O
and	O
nodes	O
.	O
</s>
<s>
For	O
the	O
benefit	O
of	O
reducing	O
network	O
complexity	O
,	O
the	O
intelligence	O
in	O
the	O
network	O
is	O
located	O
in	O
the	O
end	B-Protocol
nodes	I-Protocol
.	O
</s>
<s>
As	O
a	O
consequence	O
of	O
this	O
design	O
,	O
the	O
Internet	B-Protocol
Protocol	I-Protocol
only	O
provides	O
best-effort	O
delivery	O
and	O
its	O
service	O
is	O
characterized	O
as	O
unreliable	B-Protocol
.	O
</s>
<s>
In	O
network	O
architectural	O
parlance	O
,	O
it	O
is	O
a	O
connectionless	B-Protocol
protocol	I-Protocol
,	O
in	O
contrast	O
to	O
connection-oriented	B-Protocol
communication	I-Protocol
.	O
</s>
<s>
Various	O
fault	O
conditions	O
may	O
occur	O
,	O
such	O
as	O
data	O
corruption	O
,	O
packet	B-Protocol
loss	I-Protocol
and	O
duplication	O
.	O
</s>
<s>
Because	O
routing	B-Protocol
is	O
dynamic	O
,	O
meaning	O
every	O
packet	B-Protocol
is	O
treated	O
independently	O
,	O
and	O
because	O
the	O
network	O
maintains	O
no	O
state	O
based	O
on	O
the	O
path	O
of	O
prior	O
packets	B-Protocol
,	O
different	O
packets	B-Protocol
may	O
be	O
routed	B-Protocol
to	O
the	O
same	O
destination	O
via	O
different	O
paths	O
,	O
resulting	O
in	O
out-of-order	B-Protocol
delivery	I-Protocol
to	O
the	O
receiver	O
.	O
</s>
<s>
All	O
fault	O
conditions	O
in	O
the	O
network	O
must	O
be	O
detected	O
and	O
compensated	O
by	O
the	O
participating	O
end	B-Protocol
nodes	I-Protocol
.	O
</s>
<s>
The	O
upper	B-Protocol
layer	I-Protocol
protocols	I-Protocol
of	O
the	O
Internet	B-Protocol
protocol	I-Protocol
suite	I-Protocol
are	O
responsible	O
for	O
resolving	O
reliability	O
issues	O
.	O
</s>
<s>
For	O
example	O
,	O
a	O
host	B-Device
may	O
buffer	B-General_Concept
network	O
data	O
to	O
ensure	O
correct	O
ordering	O
before	O
the	O
data	O
is	O
delivered	O
to	O
an	O
application	O
.	O
</s>
<s>
IPv4	B-Protocol
provides	O
safeguards	O
to	O
ensure	O
that	O
the	O
header	B-General_Concept
of	O
an	O
IP	O
packet	B-Protocol
is	O
error-free	O
.	O
</s>
<s>
A	O
routing	B-Protocol
node	O
discards	O
packets	B-Protocol
that	O
fail	O
a	O
header	B-Algorithm
checksum	I-Algorithm
test	O
.	O
</s>
<s>
Although	O
the	O
Internet	B-Protocol
Control	I-Protocol
Message	I-Protocol
Protocol	I-Protocol
(	O
ICMP	O
)	O
provides	O
notification	O
of	O
errors	O
,	O
a	O
routing	B-Protocol
node	O
is	O
not	O
required	O
to	O
notify	O
either	O
end	B-Protocol
node	I-Protocol
of	O
errors	O
.	O
</s>
<s>
IPv6	B-Protocol
,	O
by	O
contrast	O
,	O
operates	O
without	O
header	B-Algorithm
checksums	I-Algorithm
,	O
since	O
current	O
link	B-Protocol
layer	I-Protocol
technology	O
is	O
assumed	O
to	O
provide	O
sufficient	O
error	O
detection	O
.	O
</s>
<s>
One	O
of	O
the	O
technical	O
constraints	O
is	O
the	O
size	O
of	O
data	B-Protocol
packets	I-Protocol
possible	O
on	O
a	O
given	O
link	O
.	O
</s>
<s>
Facilities	O
exist	O
to	O
examine	O
the	O
maximum	B-Protocol
transmission	I-Protocol
unit	I-Protocol
(	O
MTU	B-Protocol
)	O
size	O
of	O
the	O
local	O
link	O
and	O
Path	B-Protocol
MTU	I-Protocol
Discovery	I-Protocol
can	O
be	O
used	O
for	O
the	O
entire	O
intended	O
path	O
to	O
the	O
destination	O
.	O
</s>
<s>
The	O
IPv4	B-Protocol
internetworking	B-Architecture
layer	O
automatically	O
fragments	B-Protocol
a	O
datagram	B-Protocol
into	O
smaller	O
units	O
for	O
transmission	O
when	O
the	O
link	O
MTU	B-Protocol
is	O
exceeded	O
.	O
</s>
<s>
IP	O
provides	O
re-ordering	O
of	O
fragments	B-Protocol
received	O
out	O
of	O
order	O
.	O
</s>
<s>
An	O
IPv6	B-Protocol
network	O
does	O
not	O
perform	O
fragmentation	O
in	O
network	O
elements	O
,	O
but	O
requires	O
end	O
hosts	O
and	O
higher-layer	O
protocols	O
to	O
avoid	O
exceeding	O
the	O
path	B-Protocol
MTU	I-Protocol
.	O
</s>
<s>
The	O
Transmission	B-Protocol
Control	I-Protocol
Protocol	I-Protocol
(	O
TCP	O
)	O
is	O
an	O
example	O
of	O
a	O
protocol	O
that	O
adjusts	O
its	O
segment	O
size	O
to	O
be	O
smaller	O
than	O
the	O
MTU	B-Protocol
.	O
</s>
<s>
The	O
User	B-Protocol
Datagram	I-Protocol
Protocol	I-Protocol
(	O
UDP	B-Protocol
)	O
and	O
ICMP	O
disregard	O
MTU	B-Protocol
size	O
,	O
thereby	O
forcing	O
IP	O
to	O
fragment	O
oversized	O
datagrams	B-Protocol
.	O
</s>
<s>
Consequently	O
,	O
many	O
Internet	B-Protocol
protocols	I-Protocol
exhibited	O
vulnerabilities	O
highlighted	O
by	O
network	O
attacks	O
and	O
later	O
security	O
assessments	O
.	O
</s>
