Within the Internet, an autonomous system (AS) is a collection of connected Internet Protocol (IP) routingprefixes under the control of one or more network operators on behalf of a single administrative entity or domain that presents a common, clearly defined routing policy to the Internet.
Originally the definition required control by a single entity, typically an Internet service provider or a very large organization with independent connections to multiple networks, that adhere to a single and clearly defined routing policy, as originally defined in RFC 1771. The newer definition in RFC 1930 came into use because multiple organizations can run Border Gateway Protocol (BGP) using private AS numbers to an ISP that connects all those organizations to the Internet. Even though there may be multiple autonomous systems supported by the ISP, the Internet only sees the routing policy of the ISP. That ISP must have an officially registered autonomous system number (ASN).
A unique ASN is allocated to each AS for use in BGP routing. AS numbers are important because the ASN uniquely identifies each network on the Internet.
Until 2007, AS numbers were defined as 16-bit integers, which allowed for a maximum of 65,536 assignments. RFC 4893 introduced 32-bit AS numbers, which Internet Assigned Numbers Authority (IANA) has begun to allocate to regional Internet registries (RIRs), although this proposed standard has now been replaced by RFC 6793. These numbers are written preferably as simple integers (in a notation sometimes referred to as "asplain") ranging from 0 to 4,294,967,295, or in the form called "asdot" which looks like x.y, where x and y are 16-bit numbers. Numbers of the form 0.y are exactly the old 16-bit AS numbers. The accepted textual representation of autonomous system numbers is defined in RFC 5396 as "asplain". The special 16-bit ASN 23456 ("AS_TRANS") was assigned by IANA as a placeholder for 32-bit ASN values for the case when 32-bit-ASN capable routers ("new BGP speakers") send BGP messages to routers with older BGP software ("old BGP speakers") which do not understand the new 32-bit ASNs.
The first and last ASNs of the original 16-bit integers, namely 0 and 65,535, and the last ASN of the 32-bit numbers, namely 4,294,967,295 are reserved and should not be used by operators. ASNs 64,496 to 64,511 of the original 16-bit range and 65,536 to 65,551 of the 32-bit range are reserved for use in documentation by RFC 5398. ASNs 64,512 to 65,534 of the original 16-bit AS range, and 4,200,000,000 to 4,294,967,294 of the 32-bit range are reserved for Private Use by RFC 6996, meaning they can be used internally but should not be announced to the global Internet. All other ASNs are subject to assignment by IANA.
The number of unique autonomous networks in the routing system of the Internet exceeded 5000 in 1999, 30000 in late 2008, 35000 in mid-2010, 42000 in late 2012 and 54000 in mid-2016. 
AS numbers are assigned in blocks by the Internet Assigned Numbers Authority (IANA) to regional Internet registries (RIRs). The appropriate RIR then assigns AS numbers to entities within its designated area from the block assigned by the IANA. Entities wishing to receive an ASN must complete the application process of their local RIR and be approved before being assigned an ASN. Current IANA ASN assignments to RIRs can be found on the IANA website.
- APNIC specific data can be found on a daily published textfile
- RIPE NCC specific data can be found on a daily published textfile
- AFRINIC specific data can be found on a daily published textfile
- ARIN specific data can be found on a daily published textfile
Autonomous systems can be grouped into four categories, depending on their connectivity and operating policy.
A multihomed autonomous system is an AS that maintains connections to more than one other AS. This allows the AS to remain connected to the Internet in the event of a complete failure of one of their connections. However, unlike a transit AS, this type of AS would not allow traffic from one AS to pass through on its way to another AS.
A stub autonomous system refers to an AS that is connected to only one other AS. This may be an apparent waste of an AS number if the network's routing policy is the same as its upstream AS's. However, the stub AS may, in fact, have peering with other autonomous systems that is not reflected in public route-view servers. Specific examples include private interconnections in the financial and transportation sectors.
A transit autonomous system is an AS that provides connections through itself to other networks. That is, network A can use network B, the transit AS, to connect to network C. If one AS is an ISP for another, then the former is a transit AS.[clarification needed]
An Internet Exchange Point autonomous system (IX or IXP) is a physical infrastructure through which Internet service providers (ISPs) or content delivery networks (CDNs) exchange Internet traffic between their networks (autonomous systems). Usually Internet Exchange Point ASNs are transparent.[clarification needed]
- What is an Autonomous System (AS)?
- When should an AS be created?
- What is an Autonomous System Number (ASN)?
- What is the current APNIC policy for AS assignments?
- When is a Public AS Number required?
- Is my organization eligible for a Public AS Number?
- When can I use a Private AS Number?
- I plan to change my upstream providers. Can I take my AS Number with me?
- I obtained my AS Number from my partner (which is an LIR) and they are going out of business. What happens to my AS Number?
- The organization I work for is merging with another. Can my AS Number be transferred?
What is an Autonomous System (AS)?
An AS is a group of IP networks operated by one or more network operator(s) that has a single and clearly defined external routing policy.
Exterior routing protocols are used to exchange routing information between Autonomous Systems.
For more information, see RFC 1930.
When should an AS be created?
An AS needs to be created if a network connects to more than one AS with different routing policies.
Some common examples of Autonomous Systems are networks connected to two or more upstream service providers or exchange points and networks peering locally at exchange points.
What is an Autonomous System Number (ASN)?
A public AS has a globally unique number, an AS Number, associated with it. This number is used both in the exchange of exterior routing information (between neighboring Autonomous Systems) and as an identifier of the AS itself.
There are two types of AS Numbers:
- Public AS Numbers
- Private AS Numbers
What is the current APNIC policy for AS assignments?
On 1 Jan 2010, APNIC ceased to make any distinction between two-byte and four-byte when assigning AS Numbers.
For more information, see Policies for Autonomous System number management in the Asia Pacific region.
When is a Public AS Number required?
A Public AS Number is required only when an AS is exchanging routing information with other Autonomous Systems on the public Internet. That is, all routes originating from an AS is visible on the Internet.
Is my organization eligible for a Public AS Number?
Your organization is eligible for an AS Number assignment if:
- it is currently multihomed, or
- it holds previously-allocated provider independent address space and intends to multihome in the future.
An organization will also be eligible if it can demonstrate that it will meet the above criteria upon receiving an AS Number (or within a reasonably short time afterwards).
When can I use a Private AS Number?
A Private AS Number should be used if an AS is only required to communicate via Border Gateway Protocol with a single provider. As the routing policy between the AS and the provider will not be visible in the Internet, a Private AS Number can be used for this purpose.
IANA has reserved, for Private Use, a contiguous block of 1023 Autonomous System numbers from the “16-bit Autonomous System Numbers” registry, namely 64512 – 65534 inclusive.
IANA has also reserved, for Private Use, a contiguous block of 94,967,295 Autonomous System numbers from the “32-bit Autonomous System Numbers” registry, namely 4200000000 – 4294967294 inclusive.
I plan to change my upstream providers. Can I take my AS Number with me?
This depends on how you received that AS Number. If you got it directly from APNIC or an NIR, then it is portable and you can take it with you to whichever providers you choose (subject to the agreement you signed with APNIC or the NIR).
However, if you got your AS Number from a Local Internet Registry (LIR), you can only use it while you continue to receive connectivity from the LIR. That is, if you decide to no longer use the LIR as one of your upstream providers, then you have to return the AS Number. (Note: this condition only applies to AS Numbers assigned after 1 December 2002).
I obtained my AS Number from my partner (which is an LIR) and they are going out of business. What happens to my AS Number?
If the LIR from which you obtained an AS Number is about to cease trading, all AS Numbers assigned to them for use by their customers must be returned.
As one of their customers, you have two choices:
- Return the AS Number to your provider. They will return it to APNIC (or the relevant NIR)
- Contact your provider and advise them that you wish to take over custodianship of the AS Number. This will mean that you will be asked to sign an agreement with APNIC and pay the applicable membership or non-membership fees
The organization I work for is merging with another. Can my AS Number be transferred?
Yes, however the organization that is the current custodian of the AS Number will be asked to provide legal documentation of the transfer.
For more information, contact APNIC Helpdesk.