Autonomous System numbers – FAQs

  1. What is an Autonomous System (AS)?
  2. When should an AS be created?
  3. What is an Autonomous System Number (ASN)?
  4. What is the current APNIC policy for AS assignments?
  5. When is a Public AS Number required?
  6. Is my organization eligible for a Public AS Number?
  7. When can I use a Private AS Number?
  8. I plan to change my upstream providers. Can I take my AS Number with me?
  9. 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?
  10. 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.