<s>
In	O
Internet	O
routing	B-Protocol
,	O
the	O
default-free	B-Protocol
zone	I-Protocol
(	O
DFZ	O
)	O
is	O
the	O
collection	O
of	O
all	O
Internet	O
autonomous	B-Architecture
systems	I-Architecture
(	O
AS	O
)	O
that	O
do	O
not	O
require	O
a	O
default	B-Protocol
route	I-Protocol
to	O
route	O
a	O
packet	O
to	O
any	O
destination	O
.	O
</s>
<s>
Conceptually	O
,	O
DFZ	O
routers	O
have	O
a	O
"	O
complete	O
"	O
Border	B-Protocol
Gateway	I-Protocol
Protocol	I-Protocol
table	O
,	O
sometimes	O
referred	O
to	O
as	O
the	O
Internet	B-Protocol
routing	I-Protocol
table	I-Protocol
,	O
global	B-Protocol
routing	I-Protocol
table	I-Protocol
or	O
global	B-Protocol
BGP	I-Protocol
table	I-Protocol
.	O
</s>
<s>
However	O
,	O
internet	O
routing	B-Protocol
changes	O
rapidly	O
and	O
the	O
widespread	O
use	O
of	O
route	O
filtering	O
ensures	O
that	O
no	O
router	O
has	O
a	O
complete	O
view	O
of	O
all	O
routes	O
.	O
</s>
<s>
Any	O
routing	B-Protocol
table	O
created	O
would	O
look	O
different	O
from	O
the	O
perspective	O
of	O
different	O
routers	O
,	O
even	O
if	O
a	O
stable	O
view	O
could	O
be	O
achieved	O
.	O
</s>
<s>
The	O
Weekly	O
Routing	B-Protocol
Reports	O
used	O
by	O
the	O
ISP	O
community	O
come	O
from	O
the	O
Asia-Pacific	O
Network	O
Information	O
Centre	O
(	O
APNIC	O
)	O
router	O
in	O
Tokyo	O
,	O
which	O
is	O
a	O
well-connected	O
router	O
that	O
has	O
as	O
good	O
a	O
view	O
of	O
the	O
Internet	O
as	O
any	O
other	O
single	O
router	O
.	O
</s>
<s>
For	O
serious	O
routing	B-Protocol
research	O
,	O
however	O
,	O
routing	B-Protocol
information	I-Protocol
will	O
be	O
captured	O
at	O
multiple	O
well-connected	O
sites	O
,	O
including	O
high-traffic	O
ISPs	O
(	O
see	O
the	O
"	O
skitter	O
core	O
"	O
)	O
below	O
.	O
</s>
<s>
These	O
came	O
from	O
46,795	O
autonomous	B-Architecture
systems	I-Architecture
,	O
of	O
which	O
only	O
172	O
were	O
transit-only	O
and	O
35787	O
were	O
stub/origin	O
-only	O
.	O
</s>
<s>
6087	O
autonomous	B-Architecture
systems	I-Architecture
provided	O
some	O
level	O
of	O
transit	B-Application
.	O
</s>
<s>
The	O
term	O
"	O
default-free	B-Protocol
zone	I-Protocol
"	O
is	O
sometimes	O
confused	O
with	O
an	O
"	O
Internet	B-General_Concept
core	I-General_Concept
"	O
or	O
Internet	B-General_Concept
backbone	I-General_Concept
,	O
but	O
there	O
has	O
been	O
no	O
true	O
"	O
core	O
"	O
since	O
before	O
the	O
Border	B-Protocol
Gateway	I-Protocol
Protocol	I-Protocol
(	O
BGP	B-Protocol
)	O
was	O
introduced	O
.	O
</s>
<s>
In	O
pre-BGP	O
days	O
,	O
when	O
the	O
Exterior	B-Protocol
Gateway	I-Protocol
Protocol	I-Protocol
(	O
EGP	O
)	O
was	O
the	O
exterior	B-Protocol
routing	I-Protocol
protocol	I-Protocol
,	O
it	O
indeed	O
could	O
be	O
assumed	O
there	O
was	O
a	O
single	O
Internet	B-General_Concept
core	I-General_Concept
.	O
</s>
<s>
At	O
best	O
,	O
today	O
's	O
definition	O
of	O
the	O
Internet	B-General_Concept
core	I-General_Concept
is	O
statistical	O
,	O
with	O
the	O
"	O
skitter	O
core	O
"	O
being	O
some	O
number	O
of	O
AS	O
with	O
the	O
greatest	O
traffic	O
according	O
to	O
the	O
CAIDA	O
measurements	O
,	O
previously	O
made	O
with	O
its	O
measuring	O
tool	O
called	O
"	O
skitter	O
"	O
.	O
</s>
<s>
Large	O
Internet	B-Protocol
Exchange	I-Protocol
Points	I-Protocol
(	O
IXP	B-Protocol
)	O
—	O
in	O
that	O
they	O
typically	O
include	O
full	O
routes	O
as	O
seen	O
by	O
multiple	O
ISPs	O
,	O
as	O
well	O
as	O
customer	O
routes	O
,	O
in	O
their	O
exchange	O
fabric	O
—	O
are	O
extremely	O
good	O
places	O
to	O
assess	O
global	O
Internet	O
routing	B-Protocol
.	O
</s>
<s>
Before	O
the	O
current	O
commercial	O
Internet	O
evolved	O
,	O
the	O
NSFNET	O
,	O
which	O
interconnected	O
five	O
US	O
government	O
funded	O
supercomputer	O
centers	O
,	O
could	O
have	O
been	O
considered	O
the	O
high-speed	O
Internet	B-General_Concept
core	I-General_Concept
.	O
</s>
<s>
Four	O
IXPs	B-Protocol
supported	O
NSFNET	O
,	O
but	O
these	O
IXPs	B-Protocol
evolved	O
into	O
a	O
model	O
where	O
commercial	O
traffic	O
could	O
meet	O
there	O
.	O
</s>
<s>
It	O
is	O
common	O
practice	O
,	O
in	O
a	O
multihomed	B-Protocol
but	O
stub	B-Architecture
(	O
i.e.	O
,	O
non-transit	O
)	O
autonomous	B-Architecture
system	I-Architecture
,	O
for	O
the	O
BGP-speaking	O
router(s )	O
to	O
take	O
"	O
full	O
routes	O
"	O
from	O
the	O
various	O
ISPs	O
to	O
which	O
the	O
AS	O
is	O
multihomed	B-Protocol
.	O
</s>
