<s>
An	O
IP	B-Protocol
tunnel	I-Protocol
is	O
an	O
Internet	B-Protocol
Protocol	I-Protocol
(	O
IP	O
)	O
network	O
communications	O
channel	O
between	O
two	O
networks	O
.	O
</s>
<s>
It	O
is	O
used	O
to	O
transport	O
another	O
network	O
protocol	O
by	O
encapsulation	B-Protocol
of	O
its	O
packets	B-Protocol
.	O
</s>
<s>
IP	B-Protocol
tunnels	I-Protocol
are	O
often	O
used	O
for	O
connecting	O
two	O
disjoint	O
IP	O
networks	O
that	O
do	O
n't	O
have	O
a	O
native	O
routing	O
path	O
to	O
each	O
other	O
,	O
via	O
an	O
underlying	O
routable	O
protocol	O
across	O
an	O
intermediate	O
transport	O
network	O
.	O
</s>
<s>
In	O
conjunction	O
with	O
the	O
IPsec	B-Protocol
protocol	O
they	O
may	O
be	O
used	O
to	O
create	O
a	O
virtual	B-Application
private	I-Application
network	I-Application
between	O
two	O
or	O
more	O
private	O
networks	O
across	O
a	O
public	O
network	O
such	O
as	O
the	O
Internet	O
.	O
</s>
<s>
Another	O
prominent	O
use	O
is	O
to	O
connect	O
islands	O
of	O
IPv6	B-Protocol
installations	O
across	O
the	O
IPv4	B-Protocol
Internet	I-Protocol
.	O
</s>
<s>
In	O
IP	B-Protocol
tunnelling	I-Protocol
,	O
every	O
IP	O
packet	B-Protocol
,	O
including	O
addressing	O
information	O
of	O
its	O
source	O
and	O
destination	O
IP	O
networks	O
,	O
is	O
encapsulated	O
within	O
another	O
packet	B-Protocol
format	O
native	O
to	O
the	O
transit	O
network	O
.	O
</s>
<s>
At	O
the	O
borders	O
between	O
the	O
source	O
network	O
and	O
the	O
transit	O
network	O
,	O
as	O
well	O
as	O
the	O
transit	O
network	O
and	O
the	O
destination	O
network	O
,	O
gateways	O
are	O
used	O
that	O
establish	O
the	O
end-points	O
of	O
the	O
IP	B-Protocol
tunnel	I-Protocol
across	O
the	O
transit	O
network	O
.	O
</s>
<s>
Thus	O
,	O
the	O
IP	B-Protocol
tunnel	I-Protocol
endpoints	O
become	O
native	O
IP	O
routers	O
that	O
establish	O
a	O
standard	O
IP	O
route	O
between	O
the	O
source	O
and	O
destination	O
networks	O
.	O
</s>
<s>
Packets	B-Protocol
traversing	O
these	O
end-points	O
from	O
the	O
transit	O
network	O
are	O
stripped	O
from	O
their	O
transit	O
frame	O
format	O
headers	O
and	O
trailers	O
used	O
in	O
the	O
tunnelling	B-Protocol
protocol	I-Protocol
and	O
thus	O
converted	O
into	O
native	O
IP	O
format	O
and	O
injected	O
into	O
the	O
IP	O
stack	O
of	O
the	O
tunnel	O
endpoints	O
.	O
</s>
<s>
In	O
addition	O
,	O
any	O
other	O
protocol	O
encapsulations	B-Protocol
used	O
during	O
transit	O
,	O
such	O
as	O
IPsec	B-Protocol
or	O
Transport	B-Protocol
Layer	I-Protocol
Security	I-Protocol
,	O
are	O
removed	O
.	O
</s>
<s>
IP	B-Protocol
in	I-Protocol
IP	I-Protocol
,	O
sometimes	O
called	O
ipencap	O
,	O
is	O
an	O
example	O
of	O
IP	O
encapsulation	B-Protocol
within	O
IP	O
and	O
is	O
described	O
in	O
RFC	O
2003	O
.	O
</s>
<s>
Other	O
variants	O
of	O
the	O
IP-in-IP	O
variety	O
are	O
IPv6-in-IPv4	O
(	O
6in4	B-Protocol
)	O
and	O
IPv4-in-IPv6	O
(	O
4in6	B-Protocol
)	O
.	O
</s>
<s>
IP	B-Protocol
tunneling	I-Protocol
often	O
bypasses	O
simple	O
firewall	B-Application
rules	O
transparently	O
since	O
the	O
specific	O
nature	O
and	O
addressing	O
of	O
the	O
original	O
datagrams	O
are	O
hidden	O
.	O
</s>
<s>
Content-control	B-Protocol
software	I-Protocol
is	O
usually	O
required	O
to	O
block	O
IP	B-Protocol
tunnels	I-Protocol
.	O
</s>
<s>
The	O
first	O
specification	O
of	O
IP	B-Protocol
tunneling	I-Protocol
was	O
in	O
RFC	O
1075	O
,	O
which	O
described	O
DVMRP	B-Protocol
,	O
the	O
first	O
IP	O
multicast	O
routing	O
protocol	O
.	O
</s>
<s>
Because	O
multicast	O
used	O
special	O
IPv4	B-Protocol
addresses	O
,	O
testing	O
DVMRP	B-Protocol
required	O
a	O
way	O
to	O
get	O
IP	O
datagrams	O
across	O
portions	O
of	O
the	O
Internet	O
that	O
did	O
not	O
yet	O
recognize	O
multicast	O
addresses	O
.	O
</s>
<s>
This	O
was	O
solved	O
by	O
IP	B-Protocol
tunneling	I-Protocol
.	O
</s>
<s>
The	O
first	O
approach	O
to	O
IP	B-Protocol
tunneling	I-Protocol
used	O
an	O
IP	O
Loose	O
Source	O
Route	O
and	O
Record	O
(	O
LSRR	O
)	O
Option	O
to	O
hide	O
the	O
multicast	O
address	O
from	O
the	O
non-multicast	O
aware	O
routers	O
.	O
</s>
<s>
A	O
multicast-aware	O
destination	O
router	O
would	O
remove	O
the	O
LSRR	O
option	O
from	O
the	O
packet	B-Protocol
and	O
restore	O
the	O
multicast	O
IP	O
address	O
to	O
the	O
packet	B-Protocol
's	O
IP	O
destination	O
field	O
.	O
</s>
<s>
The	O
other	O
approach	O
in	O
the	O
DVMRP	B-Protocol
specification	O
was	O
IP	B-Protocol
in	I-Protocol
IP	I-Protocol
,	O
as	O
described	O
above	O
.	O
</s>
<s>
IP	B-Protocol
in	I-Protocol
IP	I-Protocol
soon	O
became	O
the	O
preferred	O
approach	O
,	O
and	O
was	O
later	O
put	O
to	O
use	O
in	O
the	O
Mbone	B-Protocol
.	O
</s>
