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Autonomous System numbers - FAQs



Frequently Asked Questions

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 ASes.

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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 ASes are networks connected to two or more upstream service providers or exchange points and networks peering locally at exchange points.

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What is an Autonomous System Number (ASN)?

A public AS has a globally unique number, an ASN, associated with it. This number is used both in the exchange of exterior routing information (between neighboring ASes) and as an identifier of the AS itself.
There are two types of ASNs:

  • Public ASNs
  • Private ASNs

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What is the current APNIC policy for ASN assignments?

On 1 Jan 2010, APNIC ceased to make any distinction between two-byte and four-byte when assigning AS Numbers.

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Where can I learn more about 4-byte ASN?

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When is a Public ASN required?

A Public ASN is required only when an AS is exchanging routing information with other ASes on the public Internet. That is, all routes originating from an AS is visible on the Internet.

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Am I eligible for a Public ASN?

An organization is eligible for an ASN assignment if it:

  • Is multihomed (this includes organizations connected to a public Internet Exchange Point)
  • Has a single, clearly defined routing policy that is different from its providers routing policies.

An organization will also be eligible if it can demonstrate that it will meet the above criteria upon receiving an ASN (or within a reasonably short time thereafter).

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When can I use a Private ASN?

A Private ASN should be used if an AS is only required to communicate via BGP with a single provider. As the routing policy between the AS and the provider will not be visible in the Internet, a Private ASN can be used for this purpose.

IANA has reserved AS64512 through to AS65535 to be used as private ASNs.

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I plan to change my upstream providers - can I take my ASN with me?

This depends on how you received that ASN. 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 ASN from an 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 ASN. (Note this condition only applies to ASNs assigned after 1 December 2002).

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I obtained my ASN from my partner (which is an LIR) and they are going out of business. What happens to my ASN?

If the LIR from which you obtained an ASN is about to case trading, all ASNs assigned to them for use by their customers must be returned. As one of their customers, you have two choices:

  • Return the ASN 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 ASN. This will mean that you will be asked to sign an agreement with APNIC and pay the applicable membership or non-membership fees.

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This organization I work for is merging with another.  Can my ASN be transferred?

Yes, however the organization that is the current custodian of the ASN will be asked to provide legal documentation of the transfer.

  • Contact Helpdesk for more information.

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