BGP Public and Private AS Cheat Sheet

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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