Autonomous System or AS is a set of routable IP prefixes that belongs to a network or collections of networks managed by a single entity such as ISP, corporate network, university, etc. and maintain a single or common routing policy. Each AS has to be represented by unique number called Autonomous System Number or ASN to control and exchange routing information with others.
Two formats of ASNs:
2-byte ASN (16-bit number) –> provides for 65,536 ASNS (0 to 65535) . IANA reserved 64512 to 65534 for private use, total of 1,023 ASN, see table below
| Number | Description | RFC |
| 0 | Reserved for RPKI unallocated space | RFC7607 |
| 1 – 23455 | Public ASNs | |
| 23456 | Reserved for AS Pool Transition | RFC6793 |
| 23457 – 64495 | Public ASNs | |
| 64496 – 64511 | Reserved for use in documentation | RFC5398 |
| 64512 – 65534 | Private ASNs | RFC6996 |
| 65535 | Reserved | RFC7300 |
4-byte ASN (32-bit number) -> provides for 232 or 4,294,967,296 ASNs (0 to 4294967295). IANA reserves 94,967,295 ASNs for private use, see table below.
| Number | Description | RFC |
| 65536 – 65551 | Reserved for use in documentation | RFC5398 |
| 65552 – 131071 | Reserved |
|
| 131072 – 4199999999 | Public ASNs |
|
| 4200000000 – 4294967294 | Private ASNs | RFC6996 |
| 4294967295 | Reserverd | RFC7300 |
For full details of AS Numbers, can refer to Internet Assigned Numbers Authority ( IANA) website –> https://www.iana.org/assignments/as-numbers/as-numbers.xhtml
As per IANA website:
Autonomous System (AS) Numbers are used by various routing protocols. IANA allocates AS Numbers to Regional Internet Registries (RIRs). The RIRs further allocate or assign AS Numbers to network operators in line with RIR policies. The [five RIRs] are:
– [AFRINIC]
– [APNIC]
– [ARIN]
– [LACNIC]
– [RIPE_NCC]
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