<s>
An	O
autonomous	B-Architecture
system	I-Architecture
(	O
AS	O
)	O
is	O
a	O
collection	O
of	O
connected	O
Internet	B-Protocol
Protocol	I-Protocol
(	O
IP	O
)	O
routing	B-Protocol
prefixes	O
under	O
the	O
control	O
of	O
one	O
or	O
more	O
network	O
operators	O
on	O
behalf	O
of	O
a	O
single	O
administrative	O
entity	O
or	O
domain	O
,	O
that	O
presents	O
a	O
common	O
and	O
clearly	O
defined	O
routing	B-Protocol
policy	O
to	O
the	O
Internet	O
.	O
</s>
<s>
Each	O
AS	O
is	O
assigned	O
an	O
autonomous	B-Architecture
system	I-Architecture
number	I-Architecture
(	O
ASN	O
)	O
,	O
for	O
use	O
in	O
Border	B-Protocol
Gateway	I-Protocol
Protocol	I-Protocol
(	O
BGP	B-Protocol
)	O
routing	B-Protocol
.	O
</s>
<s>
Autonomous	B-Architecture
System	I-Architecture
Numbers	I-Architecture
are	O
assigned	O
to	O
Local	O
Internet	O
Registries	O
(	O
LIRs	O
)	O
and	O
end	O
user	O
organizations	O
by	O
their	O
respective	O
Regional	O
Internet	O
Registries	O
(	O
RIRs	O
)	O
,	O
which	O
in	O
turn	O
receive	O
blocks	O
of	O
ASNs	O
for	O
reassignment	O
from	O
the	O
Internet	O
Assigned	O
Numbers	O
Authority	O
(	O
IANA	O
)	O
.	O
</s>
<s>
Originally	O
,	O
the	O
definition	O
required	O
control	O
by	O
a	O
single	O
entity	O
,	O
typically	O
an	O
Internet	O
service	O
provider	O
(	O
ISP	O
)	O
or	O
a	O
very	O
large	O
organization	O
with	O
independent	O
connections	O
to	O
multiple	O
networks	O
,	O
that	O
adhered	O
to	O
a	O
single	O
and	O
clearly	O
defined	O
routing	B-Protocol
policy	O
.	O
</s>
<s>
In	O
March	O
1996	O
,	O
the	O
newer	O
definition	O
came	O
into	O
use	O
because	O
multiple	O
organizations	O
can	O
run	O
BGP	B-Protocol
using	O
private	O
AS	B-Architecture
numbers	I-Architecture
to	O
an	O
ISP	O
that	O
connects	O
all	O
those	O
organizations	O
to	O
the	O
Internet	O
.	O
</s>
<s>
Even	O
though	O
there	O
may	O
be	O
multiple	O
autonomous	B-Architecture
systems	I-Architecture
supported	O
by	O
the	O
ISP	O
,	O
the	O
Internet	O
only	O
sees	O
the	O
routing	B-Protocol
policy	O
of	O
the	O
ISP	O
.	O
</s>
<s>
Until	O
2007	O
,	O
AS	B-Architecture
numbers	I-Architecture
were	O
defined	O
as	O
16-bit	O
integers	O
,	O
which	O
allowed	O
for	O
a	O
maximum	O
of	O
65,536	O
assignments	O
.	O
</s>
<s>
Since	O
then	O
,	O
the	O
IANA	O
has	O
begun	O
to	O
also	O
assign	O
32-bit	O
AS	B-Architecture
numbers	I-Architecture
to	O
regional	O
Internet	O
registries	O
(	O
RIRs	O
)	O
.	O
</s>
<s>
Numbers	O
of	O
the	O
form	O
0.y	O
are	O
exactly	O
the	O
old	O
16-bit	O
AS	B-Architecture
numbers	I-Architecture
.	O
</s>
<s>
The	O
special	O
16-bit	O
ASN	O
23456	O
(	O
"	O
AS_TRANS	O
"	O
)	O
was	O
assigned	O
by	O
IANA	O
as	O
a	O
placeholder	O
for	O
32-bit	O
ASN	O
values	O
for	O
the	O
case	O
when	O
32-bit-ASN	O
capable	O
routers	O
(	O
"	O
new	O
BGP	B-Protocol
speakers	O
"	O
)	O
send	O
BGP	B-Protocol
messages	O
to	O
routers	O
with	O
older	O
BGP	B-Protocol
software	O
(	O
"	O
old	O
BGP	B-Protocol
speakers	O
"	O
)	O
which	O
do	O
not	O
understand	O
the	O
new	O
32-bit	O
ASNs	O
.	O
</s>
<s>
The	O
number	O
of	O
unique	O
autonomous	O
networks	O
in	O
the	O
routing	B-Protocol
system	O
of	O
the	O
Internet	O
exceeded	O
5,000	O
in	O
1999	O
,	O
30,000	O
in	O
late	O
2008	O
,	O
35,000	O
in	O
mid-2010	O
,	O
42,000	O
in	O
late	O
2012	O
,	O
54,000	O
in	O
mid-2016	O
and	O
60,000	O
in	O
early	O
2018	O
.	O
</s>
<s>
AS	B-Architecture
numbers	I-Architecture
are	O
assigned	O
in	O
blocks	O
by	O
Internet	O
Assigned	O
Numbers	O
Authority	O
(	O
IANA	O
)	O
to	O
regional	O
Internet	O
registries	O
(	O
RIRs	O
)	O
.	O
</s>
<s>
RIRs	O
,	O
as	O
part	O
of	O
NRO	O
,	O
can	O
revoke	O
AS	B-Architecture
numbers	I-Architecture
as	O
part	O
of	O
their	O
Internet	O
governance	O
abilities	O
.	O
</s>
<s>
Autonomous	B-Architecture
systems	I-Architecture
(	O
AS	O
)	O
can	O
be	O
grouped	O
into	O
four	O
categories	O
,	O
depending	O
on	O
their	O
connectivity	O
and	O
operating	O
policy	O
.	O
</s>
<s>
multihomed	B-Protocol
:	O
An	O
AS	O
that	O
maintains	O
connections	O
to	O
more	O
than	O
one	O
other	O
AS	O
.	O
</s>
<s>
However	O
,	O
unlike	O
a	O
transit	B-Application
AS	O
,	O
this	O
type	O
of	O
AS	O
would	O
not	O
allow	O
traffic	O
from	O
one	O
AS	O
to	O
pass	O
through	O
on	O
its	O
way	O
to	O
another	O
AS	O
.	O
</s>
<s>
stub	B-Architecture
:	O
An	O
AS	O
that	O
is	O
connected	O
to	O
only	O
one	O
other	O
AS	O
.	O
</s>
<s>
This	O
may	O
be	O
an	O
apparent	O
waste	O
of	O
an	O
AS	B-Architecture
number	I-Architecture
if	O
the	O
network	O
's	O
routing	B-Protocol
policy	O
is	O
the	O
same	O
as	O
its	O
upstream	O
AS	O
's	O
.	O
</s>
<s>
However	O
,	O
the	O
stub	B-Architecture
AS	O
may	O
have	O
peering	B-Protocol
with	O
other	O
autonomous	B-Architecture
systems	I-Architecture
that	O
is	O
not	O
reflected	O
in	O
public	O
route-view	B-Application
servers	I-Application
.	O
</s>
<s>
transit	B-Application
:	O
An	O
AS	O
that	O
acts	O
as	O
a	O
router	O
between	O
two	O
ASes	O
is	O
called	O
a	O
transit	B-Application
.	O
</s>
<s>
Since	O
not	O
all	O
ASes	O
are	O
directly	O
connected	O
with	O
every	O
other	O
AS	O
,	O
a	O
transit	B-Application
AS	O
carries	O
data	O
traffic	O
between	O
one	O
AS	O
to	O
another	O
AS	O
to	O
which	O
it	O
has	O
links	O
.	O
</s>
<s>
Internet	B-Protocol
Exchange	I-Protocol
Point	I-Protocol
(	O
IX	O
or	O
IXP	B-Protocol
)	O
:	O
A	O
physical	O
infrastructure	O
through	O
which	O
ISPs	O
or	O
content	B-Operating_System
delivery	I-Operating_System
networks	I-Operating_System
(	O
CDNs	O
)	O
exchange	O
Internet	O
traffic	O
between	O
their	O
networks	O
(	O
autonomous	B-Architecture
systems	I-Architecture
)	O
.	O
</s>
<s>
These	O
are	O
often	O
groups	O
of	O
local	O
ISPs	O
that	O
band	O
together	O
to	O
exchange	O
data	O
by	O
splitting	O
the	O
costs	O
of	O
a	O
local	O
networking	O
hub	O
,	O
avoiding	O
the	O
higher	O
costs	O
(	O
and	O
bandwidth	O
charges	O
)	O
of	O
a	O
Transit	B-Application
AS	O
.	O
</s>
<s>
IXP	B-Protocol
ASNs	O
are	O
usually	O
transparent	O
.	O
</s>
<s>
By	O
having	O
presence	O
in	O
an	O
IXP	B-Protocol
,	O
ASes	O
shorten	O
transit	B-Application
path	O
to	O
other	O
participating	O
ASes	O
,	O
thereby	O
reducing	O
network	B-Protocol
latency	I-Protocol
and	O
improving	O
round-trip	O
delay	O
.	O
</s>
<s>
Autonomous	B-Architecture
systems	I-Architecture
can	O
be	O
included	O
in	O
one	O
or	O
more	O
AS-SETs	O
,	O
for	O
example	O
AS-SET	O
of	O
RIPE	O
NCC	O
"	O
AS-12655	O
"	O
has	O
AS1	O
,	O
AS2	O
and	O
AS3	O
as	O
its	O
members	O
,	O
but	O
AS1	O
is	O
also	O
included	O
in	O
other	O
sets	O
in	O
ARIN	O
(	O
AS-INCAPSULA	O
)	O
and	O
APNIC	O
(	O
AS-IMCL	O
)	O
.	O
</s>
<s>
Another	O
AS-SET	O
sources	O
can	O
be	O
RADB	O
,	O
LEVEL3	O
(	O
tier	B-Architecture
1	I-Architecture
network	I-Architecture
now	O
called	O
Lumen	B-Application
Technologies	I-Application
)	O
and	O
also	O
ARIN	O
has	O
ARIN-NONAUTH	O
source	O
of	O
AS-SETs	O
.	O
</s>
<s>
AS-SETs	O
are	O
created	O
by	O
network	O
operators	O
in	O
an	O
Internet	B-Protocol
Routing	I-Protocol
Registry	I-Protocol
(	O
IRR	O
)	O
,	O
like	O
other	O
route	O
objects	O
,	O
and	O
can	O
be	O
included	O
in	O
other	O
AS-SETs	O
and	O
even	O
form	O
cycles	O
.	O
</s>
<s>
AS-SETs	O
are	O
often	O
used	O
to	O
simplify	O
management	O
of	O
published	O
routing	B-Protocol
policies	O
.	O
</s>
<s>
A	O
routing	B-Protocol
policy	O
is	O
published	O
in	O
the	O
IRR	O
using	O
"	O
import	O
"	O
and	O
"	O
export	O
"	O
(	O
or	O
the	O
newer	O
"	O
mp-import	O
"	O
and	O
"	O
mp-export	O
"	O
)	O
attributes	O
,	O
which	O
each	O
contain	O
the	O
source	O
or	O
destination	O
AS	B-Architecture
number	I-Architecture
and	O
the	O
AS	B-Architecture
number	I-Architecture
imported	O
or	O
exported	O
.	O
</s>
<s>
Instead	O
of	O
single	O
AS	B-Architecture
numbers	I-Architecture
,	O
AS-SETs	O
can	O
be	O
referenced	O
in	O
these	O
attributes	O
,	O
which	O
simplifies	O
management	O
of	O
complex	O
routing	B-Protocol
policies	O
.	O
</s>
