Network - Autonomous systems (AS)


A autonomous systems (AS) is the first level of network on the Internet.

It is a network controlled by a single administration entity.

They are a large pool of routers run by a single organization.

Autonomous systems typically belong to:

  • ISPs (Internet Service Provider)
  • or other large high-tech organizations, such as tech companies, universities, government agencies, and scientific institutions. (For instance, stackoverflow)

Each autonomous system has an unique identifiant called an autonomous system number (ASN). As of 2018, there are approximately 64,000 ASNs in-use worldwide.



Each autonomous system wishing to exchange routing information must have a registered autonomous system number (ASN).

Internet Assigned Numbers Authority (IANA) assigns ASNs to Regional Internet Registries (RIRs), which then assigns them to ISPs and networks.

ASNs are 16 bit numbers between 1 and 65534 and 32 bit numbers between 131072 and 4294967294.

These ASNs are only required for external BGP.


Separate BGP autonomous systems dynamically exchange routing information through external BGP (eBGP) peering sessions.


You can query the owner of an AS number at the RIPE database.

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