What is Autonomous System Number (ASN)?

Autonomous System Number (ASN): Here, we are going to learn about the Autonomous System Number, history, assignment, etc.
Submitted by Anushree Goswami, on January 12, 2021

ASN: Autonomous System Number

ASN is an abbreviation of "Autonomous System Number". It is used as a distinctive number in allocation to each Autonomous System to make use in BGP routing. On the Internet, ASNs are significant for the reason that the ASN distinctively classifies each network. Various association groups can operate Border Gateway Protocol (BGP), by using private AS numbers to an ISP that joins all those association groups to the internet. Even if, there may be various autonomous systems maintained by the ISP, the internet merely perceives the routing guidelines of the ISP. The ISP required having an authoritatively registered autonomous system number (ASN). Until 2007, AS numbers were described as 16-bit integers, which are permitted for the highest limit of 65,536 assignments.

  • In the case of the unique 16-bit integers (0 and 65,535), the first and last ASNs and in the case of the 32-bit numbers (4,294,967,295), the last ASN, are set aside and supposed to not be used by operators.
  • In the case of the unique 16-bit range, ASNs 64,496 to 64,511, and in the case of the 32-bit range, ASNs 65,536 to 65, 55, are set aside for use in documentation by RFC 5398.
  • In case of the unique 16-bit AS range, ASNs 64,512 to 65,534 and in case of the 32-bit AS range, ASNs 4,200,000,000 to 4,294,967,294, are set aside for Private Use by RFC 6996, signifies that, they can be used within other than supposed to not be publicized to the worldwide Internet.
  • The entire additional ASNs are focus on assignment by IANA.

As of August 2019, the number of allocated ASNs goes beyond 92,000.


  • Autonomous System Numbers are assigned in blocks by Internet Assigned Numbers Authority (IANA) to Regional Internet Registries (RIRs).
  • The suitable RIR subsequently assigns ASNs to units inside its selected region from the block assigned by IANA.

ASN Table

An absolute table of 16-bits and 32-bits ASN are given below:

Number Bits Description Reference
016ReservedRFC1930, RFC7607
1 - 23455 16 Public ASN's
23456 16 Reserved for AS Pool Transition RFC6793
23457 - 64495 16 Public ASN's
64496 - 64511 16 Reserved for use in documentation/sample code RFC5398
64512 - 65534 16 Reserved for private use RFC1930, RFC6996
65535 16 Reserved RFC7300
65536 - 65551 32 Reserved for use in documentation and sample code RFC4893, RFC5398
65552 - 131071 32 Reserved
131072 - 4199999999 32 Public 32-bit ASN's
4200000000 - 4294967294 32 Reserved for private use RFC6996
4294967295 32 Reserved RFC7300

Reference: Autonomous system (Internet)

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