What Is Autonomous System Number?

An Autonomous System Number (ASN) is a unique identifier assigned to networks, enabling them to communicate with each other over the internet.

What Is an Autonomous System Number?

An autonomous system number (ASN) is a globally unique identifier assigned to a network or group of networks under the control of a single organization, such as an internet service provider (ISP), large enterprise, or data center operator. This identifier allows the network, known as an autonomous system (AS), to connect to and interact with other networks, facilitating structured communication across the internet.

Each AS is considered a distinct routing entity, able to define its own internal routing policies while exchanging information with other networks using standardized protocols like border gateway protocol (BGP). The ASN enables the AS to establish and advertise its routing preferences, enhancing the ability to manage traffic flow and improve network reliability, scalability, and security.

ASNs are distributed by regional internet registries (RIRs) and regulated by the Internet Assigned Numbers Authority (IANA) to maintain organized, conflict-free assignments. These numbers are essential to the internet's decentralized architecture, supporting the seamless transmission of data across independent networks worldwide.

Types of Autonomous System Numbers

Autonomous system numbers (ASNs) are categorized into several types, each serving distinct roles within network routing and connectivity.

Public ASNs

Public autonomous system numbers are unique identifiers assigned to organizations that connect to other networks over the internet, enabling interconnectivity and global data routing. These ASNs are registered with regional internet registries (RIRs) and must follow specific guidelines to ensure that the AS can be recognized and managed across the global internet. Public ASNs are commonly used by ISPs, large data centers, and multinational enterprises, as they interact with multiple external networks and require public routing visibility. With a public ASN, a network can advertise its IP addresses and routing policies, ensuring efficient and reliable data exchange with other ASes worldwide.

Private ASNs

Private autonomous system numbers are used within a closed network or for limited routing scenarios where an AS doesn’t need to interact directly with the global internet. Often assigned to internal systems, subsidiaries, or branches of a larger network, private ASNs help simplify internal routing while avoiding conflicts with globally unique ASNs. These numbers are reserved for internal use and are not advertised publicly, so they do not require registration with RIRs. This type of ASN is ideal for cases like private WANs, branch-to-headquarters connections, or networks relying on a single ISP for internet connectivity without external routing needs.

4-Byte ASNs

The introduction of 4-byte ASNs expanded the ASN space to accommodate the growing number of networks worldwide. Unlike the original 2-byte ASNs, which support up to 65,536 unique numbers, 4-byte ASNs offer a range in the billions, ensuring scalability as more organizations adopt ASNs. This transition became essential as the available 2-byte ASNs neared exhaustion due to the rapid growth of networks and the internet. Many networks today are assigned 4-byte ASNs, which operate seamlessly alongside 2-byte ASNs, ensuring compatibility and continuity in routing while offering flexibility for future network expansion.

Reserved ASNs

Reserved autonomous system numbers are set aside by IANA and are not available for public or private assignment. These ASNs are typically reserved for specific technical purposes or experimental protocols, providing identifiers that can be used in testing and specialized environments without interfering with active ASNs in use on the global internet. By reserving certain ASNs, IANA ensures that there is a structured pool available for future technological developments, protocol testing, and network innovation.

Autonomous System Number Example

An example of an autonomous system number is AS15169, which is assigned to Google. This public ASN allows Google to manage its vast network infrastructure and enables its servers to communicate efficiently with other networks worldwide. AS15169 plays a crucial role in routing traffic for Google’s global services, including search, Gmail, YouTube, and cloud offerings, ensuring that data requests are directed to the closest and most efficient Google servers. By using this ASN, Google can implement routing policies that optimize user experience by reducing latency and improving reliability for users across the globe.

Benefits of Having Autonomous System Number

Having an autonomous system number (ASN) offers organizations numerous benefits, particularly when managing complex or large-scale network infrastructures. Here are some key benefits:

• Improved routing control. An ASN provides organizations with the ability to manage their own routing policies, giving them direct control over how data traffic enters and exits their network. This autonomy allows for the optimization of traffic flow, reducing latency and improving network reliability. With control over routing paths, businesses can make adjustments based on load, performance, or other criteria, ensuring a consistent quality of service for end users.
• Enhanced network resilience. Having an ASN allows organizations to establish multiple connections with different ISPs, building redundancy into their network. This multi-homing capability ensures that if one connection fails, traffic can still be routed through alternative paths, minimizing downtime and increasing overall network resilience. This is especially valuable for businesses that require uninterrupted connectivity for critical operations.
• Efficient traffic management. ASNs allow for better traffic distribution by enabling border gateway protocol (BGP) peering with other networks. By leveraging BGP, organizations can dynamically adjust traffic paths based on network conditions, avoiding congestion and optimizing performance. This flexibility is particularly advantageous for large enterprises or data centers, where efficient traffic management is key to maintaining optimal service levels.
• Increased security and control. ASNs enable organizations to implement custom security measures and manage access controls for inbound and outbound traffic. With an ASN, network administrators can restrict access based on specific policies and prevent unwanted or malicious traffic. This level of control enhances network security and allows organizations to mitigate risks from potential cyber threats more effectively.
• Scalability for future growth. An ASN provides the foundation for scaling network operations as an organization expands. By establishing independent routing capabilities, organizations can add new locations, devices, or services to their network without relying on a third-party’s routing framework. This scalability is crucial for businesses planning long-term growth, ensuring they can adapt their network to future needs efficiently.

How Do I Get an Autonomous System Number?

To obtain an autonomous system number (ASN), organizations must follow a structured application process through their regional internet registry (RIR), which manages ASNs and IP address allocations within specific regions. Here’s how you can acquire an ASN:

1. Determine your eligibility. First, verify that you meet the requirements for an ASN. Typically, ASNs are assigned to networks with complex routing needs, such as multi-homed networks connecting to two or more ISPs, or networks with unique routing policies that need to interact with other autonomous systems.
2. Contact your regional internet registry (RIR). Identify the RIR responsible for your geographic area. The five main RIRs are ARIN (American Registry for Internet Numbers) for North America, RIPE NCC (Réseaux IP Européens Network Coordination Centre) for Europe, Middle East, and parts of Central Asia, and APNIC (Asia-Pacific Network Information Centre) for the Asia-Pacific region.
3. Complete the application. Submit an ASN application through your RIR’s online portal. The application typically requires detailed information about your network setup, routing policies, and connections to other networks. Be prepared to explain your need for an ASN and provide any required documentation.
4. Pay associated fees. Most RIRs charge a one-time setup fee along with an annual membership or maintenance fee. Costs vary by region, so review the fee schedule on your RIR’s website to understand the total expenses involved.
5. Undergo review and approval. Once submitted, your application will be reviewed by the RIR, which may request additional information to confirm your eligibility. If approved, the RIR will assign your ASN, and you will receive the details needed to configure it for your network.
6. Configure your network. After obtaining an ASN, configure your network’s routers to use border gateway protocol (BGP) for routing and to establish connections with other ASes. This step enables you to announce your network’s routes to other ISPs and Autonomous Systems, finalizing your setup for independent routing.

How Much Does an Autonomous System Number Cost?

The cost of obtaining an ASN varies depending on the regional internet registry responsible for your region and the specific fee structures they have in place. Typically, RIRs charge a one-time application or setup fee, followed by an annual maintenance or membership fee. Here's a general breakdown of typical costs by region:

• ARIN (North America). For the American Registry for Internet Numbers (ARIN), the initial setup fee for an ASN is usually around $500, with an annual maintenance fee based on membership, which can range from $150 to $2,000 depending on the size and type of IP address allocation.
• RIPE NCC (Europe, Middle East, Central Asia). RIPE NCC charges an annual membership fee, typically starting around €1,400, which includes ASN allocations. No separate setup fee for ASNs is charged, as the cost is bundled with the RIPE membership.
• APNIC (Asia-Pacific). APNIC requires an annual membership, typically costing between $1,300 and $5,500 AUD, depending on the organization’s resource needs. The ASN assignment is included within this membership fee, so no separate fee applies specifically to ASNs.
• LACNIC (Latin America and Caribbean). LACNIC charges a one-time registration fee for an ASN, around $1,000 USD, plus an annual membership fee, which is based on the organization’s membership category and can vary widely.
• AFRINIC (Africa). AFRINIC charges a one-time setup fee for ASNs, generally around $500, plus an annual membership fee starting around $1,250 USD for small organizations, with fees scaling up for larger networks.