In the Internet, an autonomous system (AS) is a collection of IP networks under control of a single entity, typically an Internet service provider or a very large organization with redundant connections to the rest of the Internet, that adhere to a single and clearly defined routing policy.
A unique AS number (or ASN) is allocated to each AS for use in BGP routing. The numbers are assigned by the same authorities that allocate IP addresses. AS numbers are currently 16-bit integers. There are public numbers, which may be used on the Internet and range from 1 to 64511, and private numbers from 64512 to 65535, which can be used within an organization.
Types of AS:
- Multihomed AS
- Stub AS
- Transit AS
AS numbers are allocated to the regional registries by the Internet Assigned Numbers Authority (IANA, http://www.iana.org). From the IANA website, http://www.iana.org/assignments/as-numbers, we get this information on which ASN blocks are assigned to which regional registries, as of February 10, 2005:
Autonomous system numbers
Number Description
------------- ------------------------------------------------------
0 Reserved - May be used to identify non-routed networks
1 - 1876 Allocated by ARIN
1877 - 1901 Allocated by RIPE NCC
1902 - 2042 Allocated by ARIN
2043 Allocated by RIPE NCC
2044 - 2046 Allocated by ARIN
2047 Allocated by RIPE NCC
2048 - 2106 Allocated by ARIN
2107 - 2136 Allocated by RIPE NCC
2137 - 2584 Allocated by ARIN
2585 - 2614 Allocated by RIPE NCC
2615 - 2772 Allocated by ARIN
2773 - 2822 Allocated by RIPE NCC
2823 - 2829 Allocated by ARIN
2830 - 2879 Allocated by RIPE NCC
2880 - 3153 Allocated by ARIN
3154 - 3353 Allocated by RIPE NCC
3354 - 4607 Allocated by ARIN
4608 - 4864 Allocated by APNIC
4865 - 5376 Allocated by ARIN
5377 - 5631 Allocated by RIPE NCC
5632 - 6655 Allocated by ARIN
6656 - 6911 Allocated by RIPE NCC
6912 - 7466 Allocated by ARIN
7467 - 7722 Allocated by APNIC
7723 - 8191 Allocated by ARIN
8192 - 9215 Allocated by RIPE NCC
9216 - 10239 Allocated by APNIC
10240 - 11263 Allocated by ARIN
11264 - 12287 Allocated by ARIN
12288 - 13311 Allocated by RIPE NCC
13312 - 14335 Allocated by ARIN
14336 - 15359 Allocated by ARIN
15360 - 16383 Allocated by RIPE NCC
16384 - 17407 Allocated by ARIN
17408 - 18431 Allocated by APNIC
18432 - 19455 Allocated by ARIN
19456 - 20479 Allocated by ARIN
20480 - 21503 Allocated by RIPE NCC
21504 - 22527 Allocated by ARIN
22528 - 23551 Allocated by ARIN
23552 - 24575 Allocated by APNIC
24576 - 25599 Allocated by RIPE NCC
25600 - 26623 Allocated by ARIN (March 2002)
26624 - 27647 Allocated by ARIN (September 2002)
27648 - 28671 Allocated by LACNIC (November 2002)
28672 - 29695 Allocated by RIPE NCC (December 2002)
29696 - 30719 Allocated by ARIN (March 2003)
30720 - 31743 Allocated by RIPE NCC (September 2003)
31744 - 32767 Allocated by ARIN (November 2003)
32768 - 33791 Allocated by ARIN (May 2004)
33792 - 34815 Allocated by RIPE NCC (June 2004)
34816 - 35839 Allocated by RIPE NCC (February 2005)
35840 - 36863 Allocated by ARIN (February 2005)
36864 - 48127 Held by the IANA
48128 - 64511 Reserved by the IANA
64512 - 65534 Designated for private use (Allocated to the IANA)
65535 Reserved
Specially designated Autonomous System Numbers:
23456 AS_TRAN [draft-ietf-idr-as4bytes-04.txt]
