ASN (Autonomous System Number) is a unique identifier assigned to an autonomous system (AS), which is a collection of IP networks and routers under the control of a single organization that presents a common routing policy to the internet.

Key Characteristics of ASN:

  1. Unique Identifier: Each ASN is a unique number assigned to an autonomous system to distinguish it from other autonomous systems.
  2. Routing Information: ASNs are used by Border Gateway Protocol (BGP) to exchange routing information between autonomous systems on the internet.
  3. Autonomous System: An autonomous system is a large network or group of networks with a unified routing policy, typically managed by a single organization, such as an internet service provider (ISP), university, or large enterprise.

Types of ASN:

  1. Public ASN: Used for autonomous systems that connect to other autonomous systems on the internet. These ASNs must be unique globally and are assigned by regional internet registries (RIRs).
  2. Private ASN: Used for autonomous systems that do not require a unique global identifier, typically used for internal networks. Private ASNs fall within the range 64512 to 65534 and 4200000000 to 4294967294.

Role of ASN in Internet Routing:

  1. BGP Routing: ASNs are essential for BGP, the protocol used to exchange routing information between different autonomous systems. BGP relies on ASNs to make routing decisions and ensure data packets are delivered efficiently.
  2. Path Selection: ASNs help in path selection by providing a way to identify the source and destination of routing information, allowing networks to choose the best paths for data transmission.
  3. Network Management: ASNs are crucial for network management, enabling organizations to manage and optimize their routing policies, improve network performance, and ensure reliable connectivity.


Consider an internet service provider (ISP) that operates a large network. This ISP is assigned an ASN, say 12345, by a regional internet registry. The ASN 12345 is used by the ISP to advertise its routing policies and network prefixes to other ISPs and organizations on the internet through BGP. This allows the ISP to establish peering relationships, exchange routing information, and ensure efficient data transmission across the internet.

ASN Allocation:

ASNs are allocated by regional internet registries (RIRs), such as:

  • ARIN (American Registry for Internet Numbers): Serves North America.
  • RIPE NCC (Réseaux IP Européens Network Coordination Centre): Serves Europe, the Middle East, and parts of Central Asia.
  • APNIC (Asia-Pacific Network Information Centre): Serves the Asia-Pacific region.
  • LACNIC (Latin American and Caribbean Internet Addresses Registry): Serves Latin America and the Caribbean.
  • AFRINIC (African Network Information Centre): Serves Africa.

In summary, ASNs are vital for the organization and operation of the internet, facilitating efficient and reliable routing of data between autonomous systems.

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