<s>
Internet	B-Protocol
Protocol	I-Protocol
version	O
4	O
(	O
IPv4	O
)	O
is	O
the	O
fourth	O
version	O
of	O
the	O
Internet	B-Protocol
Protocol	I-Protocol
(	O
IP	O
)	O
.	O
</s>
<s>
It	O
is	O
one	O
of	O
the	O
core	O
protocols	O
of	O
standards-based	O
internetworking	B-Architecture
methods	O
in	O
the	O
Internet	O
and	O
other	O
packet-switched	B-Protocol
networks	I-Protocol
.	O
</s>
<s>
It	O
is	O
still	O
used	O
to	O
route	B-Protocol
most	O
Internet	O
traffic	O
today	O
,	O
even	O
with	O
the	O
ongoing	O
deployment	O
of	O
Internet	B-Protocol
Protocol	I-Protocol
version	I-Protocol
6	I-Protocol
(	O
IPv6	B-Protocol
)	O
,	O
its	O
successor	O
.	O
</s>
<s>
IPv4	O
uses	O
a	O
32-bit	O
address	B-General_Concept
space	I-General_Concept
which	O
provides	O
4,294,967,296	O
(	O
232	O
)	O
unique	O
addresses	O
,	O
but	O
large	O
blocks	O
are	O
reserved	O
for	O
special	O
networking	O
purposes	O
.	O
</s>
<s>
Internet	B-Protocol
Protocol	I-Protocol
version	O
4	O
is	O
described	O
in	O
IETF	O
publication	O
RFC	O
791	O
(	O
September	O
1981	O
)	O
,	O
replacing	O
an	O
earlier	O
definition	O
of	O
January	O
1980	O
(	O
RFC	O
760	O
)	O
.	O
</s>
<s>
In	O
March	O
1982	O
,	O
the	O
US	O
Department	O
of	O
Defense	O
decided	O
on	O
the	O
Internet	B-Protocol
Protocol	I-Protocol
Suite	I-Protocol
(	O
TCP/IP	B-Protocol
)	O
as	O
the	O
standard	O
for	O
all	O
military	O
computer	B-Architecture
networking	I-Architecture
.	O
</s>
<s>
The	O
Internet	B-Protocol
Protocol	I-Protocol
is	O
the	O
protocol	B-Protocol
that	O
defines	O
and	O
enables	O
internetworking	B-Architecture
at	O
the	O
internet	B-Protocol
layer	I-Protocol
of	O
the	O
Internet	B-Protocol
Protocol	I-Protocol
Suite	I-Protocol
.	O
</s>
<s>
It	O
uses	O
a	O
logical	O
addressing	O
system	O
and	O
performs	O
routing	O
,	O
which	O
is	O
the	O
forwarding	O
of	O
packets	B-Protocol
from	O
a	O
source	O
host	O
to	O
the	O
next	O
router	B-Protocol
that	O
is	O
one	O
hop	B-Protocol
closer	O
to	O
the	O
intended	O
destination	O
host	O
on	O
another	O
network	O
.	O
</s>
<s>
IPv4	O
is	O
a	O
connectionless	B-Protocol
protocol	I-Protocol
,	O
and	O
operates	O
on	O
a	O
best-effort	O
delivery	O
model	O
,	O
in	O
that	O
it	O
does	O
not	O
guarantee	O
delivery	O
,	O
nor	O
does	O
it	O
assure	O
proper	O
sequencing	O
or	O
avoidance	O
of	O
duplicate	O
delivery	O
.	O
</s>
<s>
These	O
aspects	O
,	O
including	O
data	O
integrity	O
,	O
are	O
addressed	O
by	O
an	O
upper	B-Protocol
layer	I-Protocol
transport	B-Architecture
protocol	I-Architecture
,	O
such	O
as	O
the	O
Transmission	B-Protocol
Control	I-Protocol
Protocol	I-Protocol
(	O
TCP	B-Protocol
)	O
.	O
</s>
<s>
IPv4	O
uses	O
32-bit	O
addresses	O
which	O
limits	O
the	O
address	B-General_Concept
space	I-General_Concept
to	O
(	O
232	O
)	O
addresses	O
.	O
</s>
<s>
IPv4	O
reserves	O
special	O
address	O
blocks	O
for	O
private	B-Protocol
networks	I-Protocol
(	O
~	O
18	O
million	O
addresses	O
)	O
and	O
multicast	O
addresses	O
(	O
~	O
270	O
million	O
addresses	O
)	O
.	O
</s>
<s>
They	O
are	O
most	O
often	O
written	O
in	O
dot-decimal	B-Protocol
notation	I-Protocol
,	O
which	O
consists	O
of	O
four	O
octets	O
of	O
the	O
address	O
expressed	O
individually	O
in	O
decimal	B-Application
numbers	O
and	O
separated	O
by	O
periods	O
.	O
</s>
<s>
For	O
example	O
,	O
the	O
quad-dotted	O
IP	B-Protocol
address	I-Protocol
192.0.2.235	O
represents	O
the	O
32-bit	O
decimal	B-Application
number	O
3221226219	O
,	O
which	O
in	O
hexadecimal	O
format	O
is	O
0xC00002EB	O
.	O
</s>
<s>
CIDR	B-Protocol
notation	I-Protocol
combines	O
the	O
address	O
with	O
its	O
routing	O
prefix	O
in	O
a	O
compact	O
format	O
,	O
in	O
which	O
the	O
address	O
is	O
followed	O
by	O
a	O
slash	O
character	O
(	O
/	O
)	O
and	O
the	O
count	O
of	O
leading	O
consecutive	O
1	O
bits	O
in	O
the	O
routing	O
prefix	O
(	O
subnet	O
mask	O
)	O
.	O
</s>
<s>
Other	O
address	O
representations	O
were	O
in	O
common	O
use	O
when	O
classful	B-Protocol
networking	O
was	O
practiced	O
.	O
</s>
<s>
For	O
example	O
,	O
the	O
loopback	B-Protocol
address	I-Protocol
127.0.0.1	B-Protocol
is	O
commonly	O
written	O
as	O
127.1	B-Protocol
,	O
given	O
that	O
it	O
belongs	O
to	O
a	O
class-A	O
network	O
with	O
eight	O
bits	O
for	O
the	O
network	O
mask	O
and	O
24	O
bits	O
for	O
the	O
host	O
number	O
.	O
</s>
<s>
In	O
the	O
original	O
design	O
of	O
IPv4	O
,	O
an	O
IP	B-Protocol
address	I-Protocol
was	O
divided	O
into	O
two	O
parts	O
:	O
the	O
network	O
identifier	O
was	O
the	O
most	O
significant	O
octet	O
of	O
the	O
address	O
,	O
and	O
the	O
host	O
identifier	O
was	O
the	O
rest	O
of	O
the	O
address	O
.	O
</s>
<s>
To	O
overcome	O
this	O
limit	O
,	O
the	O
most-significant	O
address	O
octet	O
was	O
redefined	O
in	O
1981	O
to	O
create	O
network	O
classes	O
,	O
in	O
a	O
system	O
which	O
later	O
became	O
known	O
as	O
classful	B-Protocol
networking	O
.	O
</s>
<s>
Because	O
of	O
the	O
different	O
sizes	O
of	O
fields	O
in	O
different	O
classes	O
,	O
each	O
network	B-Protocol
class	I-Protocol
had	O
a	O
different	O
capacity	O
for	O
addressing	O
hosts	O
.	O
</s>
<s>
Dividing	O
existing	O
classful	B-Protocol
networks	I-Protocol
into	O
subnets	O
began	O
in	O
1985	O
with	O
the	O
publication	O
of	O
.	O
</s>
<s>
In	O
1993	O
,	O
based	O
on	O
this	O
work	O
,	O
introduced	O
Classless	B-Protocol
Inter-Domain	I-Protocol
Routing	I-Protocol
(	O
CIDR	B-Protocol
)	O
,	O
which	O
expressed	O
the	O
number	O
of	O
bits	O
(	O
from	O
the	O
most	O
significant	O
)	O
as	O
,	O
for	O
instance	O
,	O
,	O
and	O
the	O
class-based	O
scheme	O
was	O
dubbed	O
classful	B-Protocol
,	O
by	O
contrast	O
.	O
</s>
<s>
CIDR	B-Protocol
was	O
designed	O
to	O
permit	O
repartitioning	O
of	O
any	O
address	B-General_Concept
space	I-General_Concept
so	O
that	O
smaller	O
or	O
larger	O
blocks	O
of	O
addresses	O
could	O
be	O
allocated	O
to	O
users	O
.	O
</s>
<s>
The	O
hierarchical	O
structure	O
created	O
by	O
CIDR	B-Protocol
is	O
managed	O
by	O
the	O
Internet	O
Assigned	O
Numbers	O
Authority	O
(	O
IANA	O
)	O
and	O
the	O
regional	O
Internet	O
registries	O
(	O
RIRs	O
)	O
.	O
</s>
<s>
Each	O
RIR	O
maintains	O
a	O
publicly	O
searchable	O
WHOIS	B-Protocol
database	I-Protocol
that	O
provides	O
information	O
about	O
IP	B-Protocol
address	I-Protocol
assignments	O
.	O
</s>
<s>
The	O
Internet	O
Engineering	O
Task	O
Force	O
(	O
IETF	O
)	O
and	O
IANA	O
have	O
restricted	O
from	O
general	O
use	O
various	O
reserved	B-Protocol
IP	I-Protocol
addresses	I-Protocol
for	O
special	O
purposes	O
.	O
</s>
<s>
Notably	O
these	O
addresses	O
are	O
used	O
for	O
multicast	O
traffic	O
and	O
to	O
provide	O
addressing	O
space	O
for	O
unrestricted	O
uses	O
on	O
private	B-Protocol
networks	I-Protocol
.	O
</s>
<s>
|Used	O
for	O
local	O
communications	O
within	O
a	O
private	B-Protocol
network	I-Protocol
.	O
</s>
<s>
|Shared	O
address	O
space	O
for	O
communications	O
between	O
a	O
service	O
provider	O
and	O
its	O
subscribers	O
when	O
using	O
a	O
carrier-grade	B-Protocol
NAT	I-Protocol
.	O
</s>
<s>
|Used	O
for	O
loopback	B-Protocol
addresses	O
to	O
the	O
local	B-Protocol
host	I-Protocol
.	O
</s>
<s>
|Used	O
for	O
link-local	O
addresses	O
between	O
two	O
hosts	O
on	O
a	O
single	O
link	O
when	O
no	O
IP	B-Protocol
address	I-Protocol
is	O
otherwise	O
specified	O
,	O
such	O
as	O
would	O
have	O
normally	O
been	O
retrieved	O
from	O
a	O
DHCP	B-Protocol
server	I-Protocol
.	O
</s>
<s>
|Used	O
for	O
local	O
communications	O
within	O
a	O
private	B-Protocol
network	I-Protocol
.	O
</s>
<s>
|IETF	O
Protocol	B-Protocol
Assignments	O
.	O
</s>
<s>
|Assigned	O
as	O
TEST-NET-1	O
,	O
documentation	O
and	O
examples	O
.	O
</s>
<s>
Formerly	O
used	O
for	O
IPv6	B-Protocol
to	I-Protocol
IPv4	I-Protocol
relay	O
(	O
included	O
IPv6	B-Protocol
address	O
block	O
2002::/16	O
)	O
.	O
</s>
<s>
|Used	O
for	O
local	O
communications	O
within	O
a	O
private	B-Protocol
network	I-Protocol
.	O
</s>
<s>
|Assigned	O
as	O
TEST-NET-2	O
,	O
documentation	O
and	O
examples	O
.	O
</s>
<s>
|Assigned	O
as	O
TEST-NET-3	O
,	O
documentation	O
and	O
examples	O
.	O
</s>
<s>
|In	O
use	O
for	O
IP	B-Protocol
multicast	I-Protocol
.	O
</s>
<s>
|Assigned	O
as	O
MCAST-TEST-NET	O
,	O
documentation	O
and	O
examples	O
.	O
</s>
<s>
(	O
Former	O
Class	B-Protocol
E	I-Protocol
network	I-Protocol
.	O
)	O
</s>
<s>
|Reserved	O
for	O
the	O
"	O
limited	O
broadcast	B-Protocol
"	O
destination	O
address	O
.	O
</s>
<s>
Of	O
the	O
approximately	O
four	O
billion	O
addresses	O
defined	O
in	O
IPv4	O
,	O
about	O
18	O
million	O
addresses	O
in	O
three	O
ranges	O
are	O
reserved	O
for	O
use	O
in	O
private	B-Protocol
networks	I-Protocol
.	O
</s>
<s>
Packets	B-Protocol
addresses	O
in	O
these	O
ranges	O
are	O
not	O
routable	O
in	O
the	O
public	O
Internet	O
;	O
they	O
are	O
ignored	O
by	O
all	O
public	O
routers	B-Protocol
.	O
</s>
<s>
Therefore	O
,	O
private	O
hosts	O
cannot	O
directly	O
communicate	O
with	O
public	O
networks	O
,	O
but	O
require	O
network	B-Protocol
address	I-Protocol
translation	I-Protocol
at	O
a	O
routing	O
gateway	O
for	O
this	O
purpose	O
.	O
</s>
<s>
!	O
CIDR	B-Protocol
block	O
!	O
</s>
<s>
!	O
Address	B-General_Concept
range	I-General_Concept
!	O
</s>
<s>
|	O
24-bit	O
block||	O
10.0.0.0/8	B-Protocol
||	O
10.0.0.0	B-Protocol
–	O
10.255.255.255	O
||align	O
=	O
right|||Single	O
Class	O
A	O
.	O
</s>
<s>
|	O
20-bit	O
block||	O
172.16.0.0/12	B-Protocol
||	O
172.16.0.0	B-Protocol
–	O
172.31.255.255	O
||align	O
=	O
right|||Contiguous	O
range	O
of	O
16	O
Class	O
B	O
blocks	O
.	O
</s>
<s>
|	O
16-bit	B-Device
block||	O
192.168.0.0/16	B-Protocol
||	O
192.168.0.0	B-Protocol
–	O
192.168.255.255	O
||align	O
=	O
right|||Contiguous	O
range	O
of	O
256	O
Class	O
C	O
blocks	O
.	O
</s>
<s>
Since	O
two	O
private	B-Protocol
networks	I-Protocol
,	O
e.g.	O
,	O
two	O
branch	O
offices	O
,	O
cannot	O
directly	O
interoperate	O
via	O
the	O
public	O
Internet	O
,	O
the	O
two	O
networks	O
must	O
be	O
bridged	O
across	O
the	O
Internet	O
via	O
a	O
virtual	B-Application
private	I-Application
network	I-Application
(	O
VPN	B-Application
)	O
or	O
an	O
IP	B-Protocol
tunnel	I-Protocol
,	O
which	O
encapsulates	B-Protocol
packets	B-Protocol
,	O
including	O
their	O
headers	O
containing	O
the	O
private	O
addresses	O
,	O
in	O
a	O
protocol	B-Protocol
layer	O
during	O
transmission	O
across	O
the	O
public	O
network	O
.	O
</s>
<s>
Additionally	O
,	O
encapsulated	O
packets	B-Protocol
may	O
be	O
encrypted	O
for	O
transmission	O
across	O
public	O
networks	O
to	O
secure	O
the	O
data	O
.	O
</s>
<s>
RFC	O
3927	O
defines	O
the	O
special	O
address	O
block	O
169.254.0.0/16	O
for	O
link-local	B-Protocol
addressing	I-Protocol
.	O
</s>
<s>
These	O
addresses	O
are	O
only	O
valid	O
on	O
the	O
link	O
(	O
such	O
as	O
a	O
local	O
network	O
segment	O
or	O
point-to-point	B-Architecture
connection	I-Architecture
)	O
directly	O
connected	O
to	O
a	O
host	O
that	O
uses	O
them	O
.	O
</s>
<s>
Like	O
private	O
addresses	O
,	O
these	O
addresses	O
cannot	O
be	O
the	O
source	O
or	O
destination	O
of	O
packets	B-Protocol
traversing	O
the	O
internet	O
.	O
</s>
<s>
These	O
addresses	O
are	O
primarily	O
used	O
for	O
address	B-Protocol
autoconfiguration	I-Protocol
(	O
Zeroconf	B-Application
)	O
when	O
a	O
host	O
cannot	O
obtain	O
an	O
IP	B-Protocol
address	I-Protocol
from	O
a	O
DHCP	B-Protocol
server	I-Protocol
or	O
other	O
internal	O
configuration	O
methods	O
.	O
</s>
<s>
When	O
the	O
address	O
block	O
was	O
reserved	O
,	O
no	O
standards	O
existed	O
for	O
address	B-Protocol
autoconfiguration	I-Protocol
.	O
</s>
<s>
Microsoft	O
created	O
an	O
implementation	O
called	O
Automatic	O
Private	O
IP	B-Protocol
Addressing	I-Protocol
(	O
APIPA	B-Protocol
)	O
,	O
which	O
was	O
deployed	O
on	O
millions	O
of	O
machines	O
and	O
became	O
a	O
de	O
facto	O
standard	O
.	O
</s>
<s>
The	O
class	O
A	O
network	O
127.0.0.0	B-Protocol
(	O
classless	B-Protocol
network	I-Protocol
127.0.0.0/8	B-Protocol
)	O
is	O
reserved	O
for	O
loopback	B-Protocol
.	O
</s>
<s>
IP	O
packets	B-Protocol
whose	O
source	O
addresses	O
belong	O
to	O
this	O
network	O
should	O
never	O
appear	O
outside	O
a	O
host	O
.	O
</s>
<s>
Packets	B-Protocol
received	O
on	O
a	O
non-loopback	O
interface	O
with	O
a	O
loopback	B-Protocol
source	O
or	O
destination	O
address	O
must	O
be	O
dropped	O
.	O
</s>
<s>
It	O
is	O
used	O
as	O
a	O
local	O
broadcast	B-Protocol
address	I-Protocol
for	O
sending	O
messages	O
to	O
all	O
devices	O
on	O
the	O
subnet	O
simultaneously	O
.	O
</s>
<s>
For	O
networks	O
of	O
size/24	O
or	O
larger	O
,	O
the	O
broadcast	B-Protocol
address	I-Protocol
always	O
ends	O
in	O
255	O
.	O
</s>
<s>
For	O
example	O
,	O
in	O
the	O
subnet	O
192.168.5.0/24	O
(	O
subnet	O
mask	O
255.255.255.0	O
)	O
the	O
identifier	O
192.168.5.0	O
is	O
used	O
to	O
refer	O
to	O
the	O
entire	O
subnet	O
.	O
</s>
<s>
The	O
broadcast	B-Protocol
address	I-Protocol
of	O
the	O
network	O
is	O
192.168.5.255	O
.	O
</s>
<s>
TypeBinary	O
formDot-decimal	O
notationNetwork	O
space11000000.10101000.00000101.00000000192.168.5.0	O
Broadcast	B-Protocol
address11000000.10101000.00000101.11111111192.168.5.255In	O
red	O
,	O
is	O
shown	O
the	O
host	O
part	O
of	O
the	O
IP	B-Protocol
address	I-Protocol
;	O
the	O
other	O
part	O
is	O
the	O
network	O
prefix	O
.	O
</s>
<s>
For	O
example	O
,	O
in	O
the/16	O
subnet	O
192.168.0.0/255.255.0.0	O
,	O
which	O
is	O
equivalent	O
to	O
the	O
address	B-General_Concept
range	I-General_Concept
192.168.0.0	B-Protocol
–	O
192.168.255.255	O
,	O
the	O
broadcast	B-Protocol
address	I-Protocol
is	O
192.168.255.255	O
.	O
</s>
<s>
One	O
can	O
use	O
the	O
following	O
addresses	O
for	O
hosts	O
,	O
even	O
though	O
they	O
end	O
with	O
255	O
:	O
192.168.1.255	O
,	O
192.168.2.255	O
,	O
etc	O
.	O
</s>
<s>
Also	O
,	O
192.168.0.0	B-Protocol
is	O
the	O
network	O
identifier	O
and	O
must	O
not	O
be	O
assigned	O
to	O
an	O
interface	O
.	O
</s>
<s>
The	O
addresses	O
192.168.1.0	O
,	O
192.168.2.0	O
,	O
etc.	O
,	O
may	O
be	O
assigned	O
,	O
despite	O
ending	O
with	O
0	O
.	O
</s>
<s>
In	O
the	O
past	O
,	O
conflict	O
between	O
network	O
addresses	O
and	O
broadcast	B-Protocol
addresses	O
arose	O
because	O
some	O
software	O
used	O
non-standard	O
broadcast	B-Protocol
addresses	O
with	O
zeros	O
instead	O
of	O
ones	O
.	O
</s>
<s>
In	O
networks	O
smaller	O
than	O
,	O
broadcast	B-Protocol
addresses	O
do	O
not	O
necessarily	O
end	O
with	O
255	O
.	O
</s>
<s>
For	O
example	O
,	O
a	O
CIDR	B-Protocol
subnet	O
203.0.113.16/28	O
has	O
the	O
broadcast	B-Protocol
address	I-Protocol
203.0.113.31	O
.	O
</s>
<s>
TypeBinary	O
formDot-decimal	O
notationNetwork	O
space11001011.00000000.01110001.00010000203.0.113.16Broadcast	O
address11001011.00000000.01110001.00011111203.0.113.31In	O
red	O
,	O
is	O
shown	O
the	O
host	O
part	O
of	O
the	O
IP	B-Protocol
address	I-Protocol
;	O
the	O
other	O
part	O
is	O
the	O
network	O
prefix	O
.	O
</s>
<s>
These	O
networks	O
are	O
typically	O
used	O
for	O
point-to-point	B-Architecture
connections	O
.	O
</s>
<s>
There	O
is	O
no	O
network	O
identifier	O
or	O
broadcast	B-Protocol
address	I-Protocol
for	O
these	O
networks	O
.	O
</s>
<s>
Hosts	O
on	O
the	O
Internet	O
are	O
usually	O
known	O
by	O
names	O
,	O
e.g.	O
,	O
www.example.com	O
,	O
not	O
primarily	O
by	O
their	O
IP	B-Protocol
address	I-Protocol
,	O
which	O
is	O
used	O
for	O
routing	O
and	O
network	O
interface	O
identification	O
.	O
</s>
<s>
The	O
translation	O
between	O
addresses	O
and	O
domain	O
names	O
is	O
performed	O
by	O
the	O
Domain	B-Protocol
Name	I-Protocol
System	I-Protocol
(	O
DNS	B-Protocol
)	O
,	O
a	O
hierarchical	O
,	O
distributed	O
naming	O
system	O
that	O
allows	O
for	O
the	O
subdelegation	O
of	O
namespaces	O
to	O
other	O
DNS	B-Protocol
servers	O
.	O
</s>
<s>
A	O
unnumbered	O
point-to-point	B-Architecture
(	O
PtP	O
)	O
link	O
,	O
also	O
called	O
a	O
transit	O
link	O
,	O
is	O
a	O
link	O
that	O
does	O
n't	O
have	O
an	O
IP	B-Protocol
network	I-Protocol
or	O
subnet	O
number	O
associated	O
with	O
it	O
,	O
but	O
still	O
has	O
an	O
IP	B-Protocol
address	I-Protocol
.	O
</s>
<s>
The	O
purpose	O
of	O
a	O
transit	O
link	O
is	O
to	O
route	B-Protocol
datagrams	B-Protocol
.	O
</s>
<s>
They	O
are	O
used	O
to	O
free	O
IP	B-Protocol
addresses	I-Protocol
from	O
a	O
scarce	O
IP	B-Protocol
address	I-Protocol
space	O
or	O
to	O
reduce	O
the	O
management	O
of	O
assigning	O
IP	O
and	O
configuration	O
of	O
interfaces	O
.	O
</s>
<s>
Previously	O
,	O
every	O
link	O
needed	O
to	O
dedicated/30	O
or/31	O
subnet	O
using	O
2-4	O
IP	B-Protocol
addresses	I-Protocol
per	O
point-to-point	B-Architecture
link	I-Architecture
.	O
</s>
<s>
When	O
a	O
link	O
is	O
unnumbered	O
,	O
a	O
router-id	O
is	O
used	O
,	O
a	O
single	O
IP	B-Protocol
address	I-Protocol
borrowed	O
from	O
a	O
defined	O
(	O
normally	O
a	O
loopback	B-Protocol
)	O
interface	O
.	O
</s>
<s>
The	O
same	O
router-id	O
can	O
be	O
used	O
on	O
multiple	O
interfaces	O
.	O
</s>
<s>
The	O
main	O
market	O
forces	O
that	O
accelerated	O
address	O
depletion	O
included	O
the	O
rapidly	O
growing	O
number	O
of	O
Internet	O
users	O
,	O
who	O
increasingly	O
used	O
mobile	O
computing	O
devices	O
,	O
such	O
as	O
laptop	B-Device
computers	I-Device
,	O
personal	B-Application
digital	I-Application
assistants	I-Application
(	O
PDAs	B-Application
)	O
,	O
and	O
smart	B-Application
phones	I-Application
with	O
IP	O
data	O
services	O
.	O
</s>
<s>
network	B-Protocol
address	I-Protocol
translation	I-Protocol
,	O
removed	O
the	O
need	O
for	O
end-to-end	B-Application
principle	I-Application
.	O
</s>
<s>
By	O
the	O
mid-1990s	O
,	O
network	B-Protocol
address	I-Protocol
translation	I-Protocol
(	O
NAT	O
)	O
was	O
used	O
pervasively	O
in	O
network	O
access	O
provider	O
systems	O
,	O
along	O
with	O
strict	O
usage-based	O
allocation	O
policies	O
at	O
the	O
regional	O
and	O
local	O
Internet	O
registries	O
.	O
</s>
<s>
APNIC	O
was	O
the	O
first	O
RIR	O
to	O
exhaust	O
its	O
regional	O
pool	O
on	O
15	O
April	O
2011	O
,	O
except	O
for	O
a	O
small	O
amount	O
of	O
address	B-General_Concept
space	I-General_Concept
reserved	O
for	O
the	O
transition	B-Protocol
technologies	I-Protocol
to	O
IPv6	B-Protocol
,	O
which	O
is	O
to	O
be	O
allocated	O
under	O
a	O
restricted	O
policy	O
.	O
</s>
<s>
The	O
long-term	O
solution	O
to	O
address	O
exhaustion	O
was	O
the	O
1998	O
specification	O
of	O
a	O
new	O
version	O
of	O
the	O
Internet	B-Protocol
Protocol	I-Protocol
,	O
IPv6	B-Protocol
.	O
</s>
<s>
It	O
provides	O
a	O
vastly	O
increased	O
address	B-General_Concept
space	I-General_Concept
,	O
but	O
also	O
allows	O
improved	O
route	B-Protocol
aggregation	O
across	O
the	O
Internet	O
,	O
and	O
offers	O
large	O
subnetwork	O
allocations	O
of	O
a	O
minimum	O
of	O
264	O
host	O
addresses	O
to	O
end	O
users	O
.	O
</s>
<s>
However	O
,	O
IPv4	O
is	O
not	O
directly	O
interoperable	O
with	O
IPv6	B-Protocol
,	O
so	O
that	O
IPv4-only	O
hosts	O
cannot	O
directly	O
communicate	O
with	O
IPv6-only	O
hosts	O
.	O
</s>
<s>
With	O
the	O
phase-out	O
of	O
the	O
6bone	B-Protocol
experimental	O
network	O
starting	O
in	O
2004	O
,	O
permanent	O
formal	O
deployment	O
of	O
IPv6	B-Protocol
commenced	O
in	O
2006	O
.	O
</s>
<s>
Completion	O
of	O
IPv6	B-Architecture
deployment	I-Architecture
is	O
expected	O
to	O
take	O
considerable	O
time	O
,	O
so	O
that	O
intermediate	O
transition	B-Protocol
technologies	I-Protocol
are	O
necessary	O
to	O
permit	O
hosts	O
to	O
participate	O
in	O
the	O
Internet	O
using	O
both	O
versions	O
of	O
the	O
protocol	B-Protocol
.	O
</s>
<s>
An	O
IP	O
packet	B-Protocol
consists	O
of	O
a	O
header	O
section	O
and	O
a	O
data	O
section	O
.	O
</s>
<s>
An	O
IP	O
packet	B-Protocol
has	O
no	O
data	O
checksum	O
or	O
any	O
other	O
footer	O
after	O
the	O
data	O
section	O
.	O
</s>
<s>
Typically	O
the	O
link	B-Protocol
layer	I-Protocol
encapsulates	B-Protocol
IP	O
packets	B-Protocol
in	O
frames	O
with	O
a	O
CRC	O
footer	O
that	O
detects	O
most	O
errors	O
,	O
many	O
transport-layer	B-Architecture
protocols	I-Architecture
carried	O
by	O
IP	O
also	O
have	O
their	O
own	O
error	O
checking	O
.	O
</s>
<s>
The	O
IPv4	O
packet	B-Protocol
header	O
consists	O
of	O
14	O
fields	O
,	O
of	O
which	O
13	O
are	O
required	O
.	O
</s>
<s>
The	O
first	O
header	O
field	O
in	O
an	O
IP	O
packet	B-Protocol
is	O
the	O
four-bit	O
version	O
field	O
.	O
</s>
<s>
An	O
example	O
is	O
Voice	B-Application
over	I-Application
IP	I-Application
(	O
VoIP	B-Application
)	O
,	O
which	O
is	O
used	O
for	O
interactive	O
voice	O
services	O
.	O
</s>
<s>
This	O
field	O
is	O
defined	O
in	O
RFC	O
3168	O
and	O
allows	O
end-to-end	O
notification	O
of	O
network	B-Protocol
congestion	I-Protocol
without	O
dropping	B-Protocol
packets	I-Protocol
.	O
</s>
<s>
This	O
16-bit	B-Device
field	O
defines	O
the	O
entire	O
packet	B-Protocol
size	O
in	O
bytes	O
,	O
including	O
header	O
and	O
data	O
.	O
</s>
<s>
All	O
hosts	O
are	O
required	O
to	O
be	O
able	O
to	O
reassemble	O
datagrams	B-Protocol
of	O
size	O
up	O
to	O
576	O
bytes	O
,	O
but	O
most	O
modern	O
hosts	O
handle	O
much	O
larger	O
packets	B-Protocol
.	O
</s>
<s>
Links	O
may	O
impose	O
further	O
restrictions	O
on	O
the	O
packet	B-Protocol
size	O
,	O
in	O
which	O
case	O
datagrams	B-Protocol
must	O
be	O
fragmented	B-Protocol
.	O
</s>
<s>
Fragmentation	O
in	O
IPv4	O
is	O
performed	O
in	O
either	O
the	O
sending	O
host	O
or	O
in	O
routers	B-Protocol
.	O
</s>
<s>
This	O
field	O
is	O
an	O
identification	O
field	O
and	O
is	O
primarily	O
used	O
for	O
uniquely	O
identifying	O
the	O
group	O
of	O
fragments	O
of	O
a	O
single	O
IP	O
datagram	B-Protocol
.	O
</s>
<s>
Some	O
experimental	O
work	O
has	O
suggested	O
using	O
the	O
ID	O
field	O
for	O
other	O
purposes	O
,	O
such	O
as	O
for	O
adding	O
packet-tracing	O
information	O
to	O
help	O
trace	O
datagrams	B-Protocol
with	O
spoofed	O
source	O
addresses	O
,	O
but	O
RFC	O
6864	O
now	O
prohibits	O
any	O
such	O
use	O
.	O
</s>
<s>
If	O
the	O
DF	O
flag	O
is	O
set	O
,	O
and	O
fragmentation	O
is	O
required	O
to	O
route	B-Protocol
the	O
packet	B-Protocol
,	O
then	O
the	O
packet	B-Protocol
is	O
dropped	O
.	O
</s>
<s>
This	O
can	O
be	O
used	O
when	O
sending	O
packets	B-Protocol
to	O
a	O
host	O
that	O
does	O
not	O
have	O
resources	O
to	O
perform	O
reassembly	O
of	O
fragments	O
.	O
</s>
<s>
It	O
can	O
also	O
be	O
used	O
for	O
path	B-Protocol
MTU	I-Protocol
discovery	I-Protocol
,	O
either	O
automatically	O
by	O
the	O
host	O
IP	O
software	O
,	O
or	O
manually	O
using	O
diagnostic	O
tools	O
such	O
as	O
ping	B-Protocol
or	O
traceroute	B-Protocol
.	O
</s>
<s>
For	O
unfragmented	O
packets	B-Protocol
,	O
the	O
MF	O
flag	O
is	O
cleared	O
.	O
</s>
<s>
For	O
fragmented	B-Protocol
packets	B-Protocol
,	O
all	O
fragments	O
except	O
the	O
last	O
have	O
the	O
MF	O
flag	O
set	O
.	O
</s>
<s>
The	O
last	O
fragment	B-Protocol
has	O
a	O
non-zero	O
Fragment	B-Protocol
Offset	O
field	O
,	O
differentiating	O
it	O
from	O
an	O
unfragmented	O
packet	B-Protocol
.	O
</s>
<s>
This	O
field	O
specifies	O
the	O
offset	O
of	O
a	O
particular	O
fragment	B-Protocol
relative	O
to	O
the	O
beginning	O
of	O
the	O
original	O
unfragmented	O
IP	O
datagram	B-Protocol
.	O
</s>
<s>
The	O
fragmentation	O
offset	O
value	O
for	O
the	O
first	O
fragment	B-Protocol
is	O
always	O
0	O
.	O
</s>
<s>
Fragments	O
are	O
specified	O
in	O
units	O
of	O
8	O
bytes	O
,	O
which	O
is	O
why	O
fragment	B-Protocol
length	O
must	O
be	O
a	O
multiple	O
of	O
8	O
.	O
</s>
<s>
Therefore	O
,	O
the	O
13-bit	O
field	O
allows	O
a	O
maximum	O
offset	O
of	O
(	O
213	O
–	O
1	O
)	O
×	O
8	O
=	O
65,528	O
bytes	O
,	O
with	O
the	O
header	O
length	O
included	O
(	O
65,528	O
+	O
20	O
=	O
65,548	O
bytes	O
)	O
,	O
supporting	O
fragmentation	O
of	O
packets	B-Protocol
exceeding	O
the	O
maximum	O
IP	O
length	O
of	O
65,535	O
bytes	O
.	O
</s>
<s>
An	O
eight-bit	O
time	B-Protocol
to	I-Protocol
live	I-Protocol
field	O
limits	O
a	O
datagram	B-Protocol
's	O
lifetime	O
to	O
prevent	O
network	O
failure	O
in	O
the	O
event	O
of	O
a	O
routing	B-Protocol
loop	I-Protocol
.	O
</s>
<s>
In	O
practice	O
,	O
the	O
field	O
is	O
used	O
as	O
a	O
hop	B-Protocol
count	I-Protocol
—	O
when	O
the	O
datagram	B-Protocol
arrives	O
at	O
a	O
router	B-Protocol
,	O
the	O
router	B-Protocol
decrements	O
the	O
TTL	O
field	O
by	O
one	O
.	O
</s>
<s>
When	O
the	O
TTL	O
field	O
hits	O
zero	O
,	O
the	O
router	B-Protocol
discards	O
the	O
packet	B-Protocol
and	O
typically	O
sends	O
an	O
ICMP	O
time	O
exceeded	O
message	O
to	O
the	O
sender	O
.	O
</s>
<s>
The	O
program	O
traceroute	B-Protocol
sends	O
messages	O
with	O
adjusted	O
TTL	O
values	O
and	O
uses	O
these	O
ICMP	O
time	O
exceeded	O
messages	O
to	O
identify	O
the	O
routers	B-Protocol
traversed	O
by	O
packets	B-Protocol
from	O
the	O
source	O
to	O
the	O
destination	O
.	O
</s>
<s>
This	O
field	O
defines	O
the	O
protocol	B-Protocol
used	O
in	O
the	O
data	O
portion	O
of	O
the	O
IP	O
datagram	B-Protocol
.	O
</s>
<s>
IANA	O
maintains	O
a	O
list	B-Protocol
of	I-Protocol
IP	I-Protocol
protocol	I-Protocol
numbers	I-Protocol
as	O
directed	O
by	O
RFC	O
790	O
.	O
</s>
<s>
The	O
16-bit	B-Device
IPv4	B-Protocol
header	I-Protocol
checksum	I-Protocol
field	O
is	O
used	O
for	O
error-checking	O
of	O
the	O
header	O
.	O
</s>
<s>
When	O
a	O
packet	B-Protocol
arrives	O
at	O
a	O
router	B-Protocol
,	O
the	O
router	B-Protocol
calculates	O
the	O
checksum	O
of	O
the	O
header	O
and	O
compares	O
it	O
to	O
the	O
checksum	O
field	O
.	O
</s>
<s>
If	O
the	O
values	O
do	O
not	O
match	O
,	O
the	O
router	B-Protocol
discards	O
the	O
packet	B-Protocol
.	O
</s>
<s>
Errors	O
in	O
the	O
data	O
field	O
must	O
be	O
handled	O
by	O
the	O
encapsulated	O
protocol	B-Protocol
.	O
</s>
<s>
Both	O
UDP	B-Protocol
and	O
TCP	B-Protocol
have	O
separate	O
checksums	O
that	O
apply	O
to	O
their	O
data	O
.	O
</s>
<s>
When	O
a	O
packet	B-Protocol
arrives	O
at	O
a	O
router	B-Protocol
,	O
the	O
router	B-Protocol
decreases	O
the	O
TTL	O
field	O
in	O
the	O
header	O
.	O
</s>
<s>
Consequently	O
,	O
the	O
router	B-Protocol
must	O
calculate	O
a	O
new	O
header	O
checksum	O
.	O
</s>
<s>
The	O
checksum	O
field	O
is	O
the	O
16	B-Device
bit	I-Device
one	O
's	O
complement	O
of	O
the	O
one	O
's	O
complement	O
sum	O
of	O
all	O
16	B-Device
bit	I-Device
words	O
in	O
the	O
header	O
.	O
</s>
<s>
This	O
32-bit	O
field	O
is	O
the	O
IPv4	B-Protocol
address	I-Protocol
of	O
the	O
sender	O
of	O
the	O
packet	B-Protocol
.	O
</s>
<s>
Note	O
that	O
this	O
address	O
may	O
be	O
changed	O
in	O
transit	O
by	O
a	O
network	B-Protocol
address	I-Protocol
translation	I-Protocol
device	O
.	O
</s>
<s>
This	O
32-bit	O
field	O
is	O
the	O
IPv4	B-Protocol
address	I-Protocol
of	O
the	O
receiver	O
of	O
the	O
packet	B-Protocol
.	O
</s>
<s>
As	O
with	O
the	O
source	O
address	O
,	O
this	O
may	O
be	O
changed	O
in	O
transit	O
by	O
a	O
network	B-Protocol
address	I-Protocol
translation	I-Protocol
device	O
.	O
</s>
<s>
The	O
options	B-Protocol
field	I-Protocol
is	O
not	O
often	O
used	O
.	O
</s>
<s>
Packets	B-Protocol
containing	O
some	O
options	O
may	O
be	O
considered	O
as	O
dangerous	O
by	O
some	O
routers	B-Protocol
and	O
be	O
blocked	O
.	O
</s>
<s>
If	O
IHL	O
is	O
greater	O
than	O
5	O
(	O
i.e.	O
,	O
it	O
is	O
from	O
6	O
to	O
15	O
)	O
it	O
means	O
that	O
the	O
options	B-Protocol
field	I-Protocol
is	O
present	O
and	O
must	O
be	O
considered	O
.	O
</s>
<s>
|Copied||1||Set	O
to	O
1	O
if	O
the	O
options	O
need	O
to	O
be	O
copied	O
into	O
all	O
fragments	O
of	O
a	O
fragmented	B-Protocol
packet	B-Protocol
.	O
</s>
<s>
!	O
Option	O
Type	O
(	O
decimal/hexadecimal	O
)	O
!	O
</s>
<s>
The	O
packet	B-Protocol
payload	O
is	O
not	O
included	O
in	O
the	O
checksum	O
.	O
</s>
<s>
Its	O
contents	O
are	O
interpreted	O
based	O
on	O
the	O
value	O
of	O
the	O
Protocol	B-Protocol
header	O
field	O
.	O
</s>
<s>
List	B-Protocol
of	I-Protocol
IP	I-Protocol
protocol	I-Protocol
numbers	I-Protocol
contains	O
a	O
complete	O
list	O
of	O
payload	O
protocol	B-Protocol
types	O
.	O
</s>
<s>
The	O
Internet	B-Protocol
Protocol	I-Protocol
enables	O
traffic	O
between	O
networks	O
.	O
</s>
<s>
The	O
design	O
accommodates	O
networks	O
of	O
diverse	O
physical	O
nature	O
;	O
it	O
is	O
independent	O
of	O
the	O
underlying	O
transmission	O
technology	O
used	O
in	O
the	O
link	B-Protocol
layer	I-Protocol
.	O
</s>
<s>
Networks	O
with	O
different	O
hardware	O
usually	O
vary	O
not	O
only	O
in	O
transmission	O
speed	O
,	O
but	O
also	O
in	O
the	O
maximum	B-Protocol
transmission	I-Protocol
unit	I-Protocol
(	O
MTU	B-Protocol
)	O
.	O
</s>
<s>
When	O
one	O
network	O
wants	O
to	O
transmit	O
datagrams	B-Protocol
to	O
a	O
network	O
with	O
a	O
smaller	O
MTU	B-Protocol
,	O
it	O
may	O
fragment	B-Protocol
its	O
datagrams	B-Protocol
.	O
</s>
<s>
In	O
IPv4	O
,	O
this	O
function	O
was	O
placed	O
at	O
the	O
Internet	B-Protocol
Layer	I-Protocol
and	O
is	O
performed	O
in	O
IPv4	O
routers	B-Protocol
limiting	O
exposure	O
to	O
these	O
issues	O
by	O
hosts	O
.	O
</s>
<s>
In	O
contrast	O
,	O
IPv6	B-Protocol
,	O
the	O
next	O
generation	O
of	O
the	O
Internet	B-Protocol
Protocol	I-Protocol
,	O
does	O
not	O
allow	O
routers	B-Protocol
to	O
perform	O
fragmentation	O
;	O
hosts	O
must	O
perform	O
Path	B-Protocol
MTU	I-Protocol
Discovery	I-Protocol
before	O
sending	O
datagrams	B-Protocol
.	O
</s>
<s>
When	O
a	O
router	B-Protocol
receives	O
a	O
packet	B-Protocol
,	O
it	O
examines	O
the	O
destination	O
address	O
and	O
determines	O
the	O
outgoing	O
interface	O
to	O
use	O
and	O
that	O
interface	O
's	O
MTU	B-Protocol
.	O
</s>
<s>
If	O
the	O
packet	B-Protocol
size	O
is	O
bigger	O
than	O
the	O
MTU	B-Protocol
,	O
and	O
the	O
Do	O
not	O
Fragment	B-Protocol
(	O
DF	O
)	O
bit	O
in	O
the	O
packet	B-Protocol
's	O
header	O
is	O
set	O
to	O
0	O
,	O
then	O
the	O
router	B-Protocol
may	O
fragment	B-Protocol
the	O
packet	B-Protocol
.	O
</s>
<s>
The	O
router	B-Protocol
divides	O
the	O
packet	B-Protocol
into	O
fragments	O
.	O
</s>
<s>
The	O
maximum	O
size	O
of	O
each	O
fragment	B-Protocol
is	O
the	O
outgoing	O
MTU	B-Protocol
minus	O
the	O
IP	O
header	O
size	O
(	O
20	O
bytes	O
minimum	O
;	O
60	O
bytes	O
maximum	O
)	O
.	O
</s>
<s>
The	O
router	B-Protocol
puts	O
each	O
fragment	B-Protocol
into	O
its	O
own	O
packet	B-Protocol
,	O
each	O
fragment	B-Protocol
packet	B-Protocol
having	O
the	O
following	O
changes	O
:	O
</s>
<s>
The	O
total	O
length	O
field	O
is	O
the	O
fragment	B-Protocol
size	O
.	O
</s>
<s>
The	O
fragment	B-Protocol
offset	O
field	O
is	O
set	O
,	O
based	O
on	O
the	O
offset	O
of	O
the	O
fragment	B-Protocol
in	O
the	O
original	O
data	O
payload	O
.	O
</s>
<s>
For	O
example	O
,	O
for	O
an	O
MTU	B-Protocol
of	O
1,500	O
bytes	O
and	O
a	O
header	O
size	O
of	O
20	O
bytes	O
,	O
the	O
fragment	B-Protocol
offsets	O
would	O
be	O
multiples	O
of	O
(	O
0	O
,	O
185	O
,	O
370	O
,	O
555	O
,	O
740	O
,	O
etc	O
.	O
</s>
<s>
It	O
is	O
possible	O
that	O
a	O
packet	B-Protocol
is	O
fragmented	B-Protocol
at	O
one	O
router	B-Protocol
,	O
and	O
that	O
the	O
fragments	O
are	O
further	O
fragmented	B-Protocol
at	O
another	O
router	B-Protocol
.	O
</s>
<s>
For	O
example	O
,	O
a	O
packet	B-Protocol
of	O
4,520	O
bytes	O
,	O
including	O
a	O
20	O
bytes	O
IP	O
header	O
is	O
fragmented	B-Protocol
to	O
two	O
packets	B-Protocol
on	O
a	O
link	O
with	O
an	O
MTU	B-Protocol
of	O
2,500	O
bytes	O
:	O
</s>
<s>
When	O
forwarded	O
to	O
a	O
link	O
with	O
an	O
MTU	B-Protocol
of	O
1,500	O
bytes	O
,	O
each	O
fragment	B-Protocol
is	O
fragmented	B-Protocol
into	O
two	O
fragments	O
:	O
</s>
<s>
Also	O
in	O
this	O
case	O
,	O
the	O
More	O
Fragments	O
bit	O
remains	O
1	O
for	O
all	O
the	O
fragments	O
that	O
came	O
with	O
1	O
in	O
them	O
and	O
for	O
the	O
last	O
fragment	B-Protocol
that	O
arrives	O
,	O
it	O
works	O
as	O
usual	O
,	O
that	O
is	O
the	O
MF	O
bit	O
is	O
set	O
to	O
0	O
only	O
in	O
the	O
last	O
one	O
.	O
</s>
<s>
And	O
of	O
course	O
,	O
the	O
Identification	O
field	O
continues	O
to	O
have	O
the	O
same	O
value	O
in	O
all	O
re-fragmented	O
fragments	O
.	O
</s>
<s>
This	O
way	O
,	O
even	O
if	O
fragments	O
are	O
re-fragmented	O
,	O
the	O
receiver	O
knows	O
they	O
have	O
initially	O
all	O
started	O
from	O
the	O
same	O
packet	B-Protocol
.	O
</s>
<s>
A	O
receiver	O
knows	O
that	O
a	O
packet	B-Protocol
is	O
a	O
fragment	B-Protocol
,	O
if	O
at	O
least	O
one	O
of	O
the	O
following	O
conditions	O
is	O
true	O
:	O
</s>
<s>
The	O
field	O
fragment	B-Protocol
offset	O
is	O
nonzero	O
,	O
which	O
is	O
true	O
for	O
all	O
fragments	O
except	O
the	O
first	O
.	O
</s>
<s>
The	O
receiver	O
identifies	O
matching	O
fragments	O
using	O
the	O
source	O
and	O
destination	O
addresses	O
,	O
the	O
protocol	B-Protocol
ID	O
,	O
and	O
the	O
identification	O
field	O
.	O
</s>
<s>
The	O
receiver	O
reassembles	O
the	O
data	O
from	O
fragments	O
with	O
the	O
same	O
ID	O
using	O
both	O
the	O
fragment	B-Protocol
offset	O
and	O
the	O
more	O
fragments	O
flag	O
.	O
</s>
<s>
When	O
the	O
receiver	O
receives	O
the	O
last	O
fragment	B-Protocol
,	O
which	O
has	O
the	O
more	O
fragments	O
flag	O
set	O
to	O
0	O
,	O
it	O
can	O
calculate	O
the	O
size	O
of	O
the	O
original	O
data	O
payload	O
,	O
by	O
multiplying	O
the	O
last	O
fragment	B-Protocol
's	O
offset	O
by	O
eight	O
and	O
adding	O
the	O
last	O
fragment	B-Protocol
's	O
data	O
size	O
.	O
</s>
<s>
When	O
the	O
receiver	O
has	O
all	O
fragments	O
,	O
they	O
can	O
be	O
reassembled	O
in	O
the	O
correct	O
sequence	O
according	O
to	O
the	O
offsets	O
to	O
form	O
the	O
original	O
datagram	B-Protocol
.	O
</s>
<s>
IP	B-Protocol
addresses	I-Protocol
are	O
not	O
tied	O
in	O
any	O
permanent	O
manner	O
to	O
networking	O
hardware	O
and	O
,	O
indeed	O
,	O
in	O
modern	O
operating	B-General_Concept
systems	I-General_Concept
,	O
a	O
network	O
interface	O
can	O
have	O
multiple	O
IP	B-Protocol
addresses	I-Protocol
.	O
</s>
<s>
In	O
order	O
to	O
properly	O
deliver	O
an	O
IP	O
packet	B-Protocol
to	O
the	O
destination	O
host	O
on	O
a	O
link	O
,	O
hosts	O
and	O
routers	B-Protocol
need	O
additional	O
mechanisms	O
to	O
make	O
an	O
association	O
between	O
the	O
hardware	O
address	O
of	O
network	O
interfaces	O
and	O
IP	B-Protocol
addresses	I-Protocol
.	O
</s>
<s>
The	O
Address	O
Resolution	O
Protocol	B-Protocol
(	O
ARP	O
)	O
performs	O
this	O
IP-address-to-hardware-address	O
translation	O
for	O
IPv4	O
.	O
</s>
<s>
For	O
example	O
,	O
unless	O
an	O
address	O
is	O
preconfigured	O
by	O
an	O
administrator	O
,	O
when	O
an	O
IP	O
host	O
is	O
booted	O
or	O
connected	O
to	O
a	O
network	O
it	O
needs	O
to	O
determine	O
its	O
IP	B-Protocol
address	I-Protocol
.	O
</s>
<s>
Protocols	O
for	O
such	O
reverse	O
correlations	O
include	O
Dynamic	B-Protocol
Host	I-Protocol
Configuration	I-Protocol
Protocol	I-Protocol
(	O
DHCP	B-Protocol
)	O
,	O
Bootstrap	B-Device
Protocol	I-Device
(	O
BOOTP	B-Device
)	O
and	O
,	O
infrequently	O
,	O
reverse	B-Protocol
ARP	I-Protocol
.	O
</s>
