<s>
An	O
IP	B-Protocol
header	I-Protocol
is	O
header	B-General_Concept
information	O
at	O
the	O
beginning	O
of	O
an	O
Internet	B-Protocol
Protocol	I-Protocol
(	O
IP	O
)	O
packet	O
.	O
</s>
<s>
An	O
IP	O
packet	O
is	O
the	O
smallest	O
message	O
entity	O
exchanged	O
via	O
the	O
Internet	B-Protocol
Protocol	I-Protocol
across	O
an	O
IP	B-Protocol
network	I-Protocol
.	O
</s>
<s>
IP	O
packets	O
consist	O
of	O
a	O
header	B-General_Concept
for	O
addressing	O
and	O
routing	O
,	O
and	O
a	O
payload	O
for	O
user	O
data	O
.	O
</s>
<s>
The	O
header	B-General_Concept
contains	O
information	O
about	O
IP	O
version	O
,	O
source	O
IP	B-Protocol
address	I-Protocol
,	O
destination	O
IP	B-Protocol
address	I-Protocol
,	O
time-to-live	B-Protocol
,	O
etc	O
.	O
</s>
<s>
The	O
payload	O
of	O
an	O
IP	O
packet	O
is	O
typically	O
a	O
datagram	B-Protocol
or	O
segment	O
of	O
the	O
higher-level	O
transport	B-Architecture
layer	I-Architecture
protocol	O
,	O
but	O
may	O
be	O
data	O
for	O
an	O
internet	B-Protocol
layer	I-Protocol
(	O
e.g.	O
,	O
ICMP	B-Protocol
or	O
ICMPv6	B-Protocol
)	O
or	O
link	B-Protocol
layer	I-Protocol
(	O
e.g.	O
,	O
OSPF	B-Protocol
)	O
instead	O
.	O
</s>
<s>
Two	O
different	O
versions	O
of	O
IP	O
are	O
used	O
in	O
practice	O
today	O
:	O
IPv4	B-Protocol
and	O
IPv6	B-Protocol
.	O
</s>
<s>
The	O
IPv6	B-Protocol
header	B-General_Concept
uses	O
IPv6	B-Protocol
addresses	I-Protocol
and	O
thus	O
offers	O
a	O
much	O
bigger	O
address	B-General_Concept
space	I-General_Concept
but	O
is	O
not	O
backward	O
compatible	O
with	O
IPv4	B-Protocol
.	O
</s>
<s>
IPv4	B-Protocol
is	O
the	O
fourth	O
version	O
in	O
the	O
development	O
of	O
the	O
Internet	B-Protocol
Protocol	I-Protocol
,	O
and	O
routes	O
most	O
traffic	O
on	O
the	O
Internet	O
.	O
</s>
<s>
The	O
IPv4	B-Protocol
header	B-General_Concept
includes	O
thirteen	O
mandatory	O
fields	O
and	O
is	O
as	O
small	O
as	O
20bytes	O
.	O
</s>
<s>
A	O
fourteenth	O
optional	O
and	O
infrequently	O
used	O
options	O
field	O
can	O
increase	O
the	O
header	B-General_Concept
size	O
.	O
</s>
<s>
IPv6	B-Protocol
is	O
the	O
successor	O
to	O
IPv4	B-Protocol
and	O
has	O
a	O
different	O
header	B-General_Concept
layout	O
.	O
</s>
<s>
It	O
was	O
defined	O
in	O
1998	O
and	O
is	O
in	O
various	O
stages	O
of	O
production	B-Application
deployment	I-Application
.	O
</s>
<s>
The	O
header	B-General_Concept
in	O
IPv6	B-Protocol
packets	O
is	O
subdivided	O
into	O
a	O
mandatory	O
fixed	O
header	B-General_Concept
and	O
optional	O
extension	O
headers	B-General_Concept
.	O
</s>
