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Autonomous System Number (ASN)

What is an Autonomous System Number (ASN)?

To understand what an ASN is or is used, we must first discuss autonomous systems (AS).  An AS comprises large network groups within the Internet, generally operated by one major entity, such as internet service providers (ISPs), multinational tech firms, schools, or government agencies.  These network groups are composed of many Internet Protocol (IP) routing prefixes that follow a specific routing policy. 

For example, a finance company might collate all their individual workers’ IP addresses under one gateway point that they can control and track.  But to have access to that single gateway point, the company has to acquire an autonomous system number (ASN).

ASN – What are they?

An ASN is a specific 16-digit identification number assigned by the Internet Assigned Numbers Authority (IANA).  An AS would need to apply for an ASN through IANA’s five regional Internet registries (RIR), depending on the AS’s location:

  • African Network Information Center (AFRINIC)
  • American Registry for Internet Numbers (ARIN)
  • Asia-Pacific Network Information Centre (APNIC)
  • Latin American and Caribbean Network Information Centre (LACNIC)
  • Réseaux IP Européens Network Coordination Centre (RIPE NCC)

An organization called Number Resource Organization (NRO) oversees the regional registries.  NRO aims to promote an open and secure Internet by maintaining registry systems and spearheading Internet governance activities and policies.

ASNs numbered 1 to 64511 are for public use, while numbers 64512 to 65535 are for private entities. Public ASNs can be used to exchange data packets or information all over the Internet.  On the other hand, a private ASN is used when data comes from one entity through Border Gateway Protocol (BGP).  BGP ensures that data packets are directed to the fastest route available.  Without BGP, data would randomly hop from one ASN to another until it gets to the correct destination.

Why do we need ASNs?

An AS communicates with another AS through BGP using their assigned ASN.  BGP requires manual configuration, which includes specifying which clusters of network an AS can share with. Without an ASN, it would be impossible to identify these network clusters. 

A group of AS connects through peering, wherein they exchange information at a local area network (LAN) called Internet exchange points (IXPs).  These physical LANs have many cables, routers, and switches.

Note that an AS would only need an ASN to communicate outside its network.  Internal routers and systems don’t need an ASN to function.

How to obtain and use an ASN?

There are specific requirements for getting an ASN.  For example, an AS has to have a distinct routing policy, a size range, and connections with other ASES.  An AS also has to submit proof that it has a multihomed site (multiple IP addresses connected to a specific network), including a list of IP addresses and ASNs of connected peers.  Again, It would be best to refer to the RIRs to learn more about each region’s requirements and fees.

This is the typical journey of a data packet using an ASN. An AS specifies a routing configuration in the BGP for how data is exchanged or communicated through their IP addresses. The BGP adds the ASN of the entity, the list of the IP addresses it owns, and its routing protocol to a global database called routing table, which determines the fastest route to deliver data from one AS to the other. The BGP uses the table as a map to refer to how data packets should move through specific networks.

What about enterprise usage of ASN?

All large firms that want to communicate over the Internet use an ASN. ASNs also allow for a secure Internet connection because ASES are confident establishing handshakes only with authorized entities. In addition, certain APIs (Application Programming Interface) can scan different ASNs to determine their identity and location.

ASNs can also be used to improve network connections. For example, ISP infrastructures use ASN to enable edge computing (the edge or limit of a particular network’s reach) at IXPs.  All ISP networks have a presence in IXPs because it allows them to share pathways or connections with other networks. Through a content delivery network (CDN) edge server, different ISP networks can use the closest network (even though they don’t own that particular network) to improve their clients’ connection latency, speed, and stability.

With the large number of networks going online every day, ASNs must be established to ensure that every data is tracked.  Regulators have an organized way of monitoring millions of communications over the Internet daily.

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