Published on Mar 3, 2023
Ghadeer Al-Mashhadi
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ASNs: Understanding Autonomous System Numbers

Autonomous System Numbers (ASNs) are unique identifiers assigned to network nodes in computer networking. They are used to identify the origin of network traffic, facilitate interconnection agreements between networks, and support routing policies and Internet topology analysis.


In the world of computer networking, an Autonomous System (AS) is a collection of connected network nodes that share a common routing policy. An Autonomous System Number (ASN) is a unique identifier assigned to an AS to identify it on the Internet. In this article, we will discuss ASNs and their significance in network infrastructure.

ASN Basics

ASNs are 32-bit numbers that are assigned by the Internet Assigned Numbers Authority (IANA) to Regional Internet Registries (RIRs) such as ARIN, RIPE NCC, APNIC, LACNIC, and AFRINIC. These RIRs then assign ASNs to Internet Service Providers (ISPs), enterprises, and other organizations that require them to operate their networks.

ASNs serve several purposes, including:

  • Identifying the origin of network traffic.
  • Facilitating interconnection agreements between ASes.
  • Supporting the development of routing policies and Internet topology analysis.

ASN Formats

ASNs are usually written in one of two formats: 2-byte ASN or 4-byte ASN.

2-byte ASNs

2-byte ASNs, also known as 16-bit ASNs, are assigned in the range 1 to 65,535. The first 64,512 ASNs (0 to 64,511) are reserved for private use and are not advertised on the public Internet. The remaining ASNs are used by ISPs and other organizations to identify their networks.

4-byte ASNs

4-byte ASNs, also known as 32-bit ASNs, were introduced to address the exhaustion of available 2-byte ASNs. 4-byte ASNs are assigned in the range 65,536 to 4,294,967,295. They are represented using the "ASPLAIN" or "ASDOT" notation.

In the ASDOT notation, 4-byte ASNs are represented as two 16-bit values separated by a dot. For example, the ASN 4200000001 would be represented as 0x1000.1.

In the ASPLAIN notation, 4-byte ASNs are represented as a single decimal value. For example, the same ASN 4200000001 would be represented as 4200000001.

ASN Peering

ASN peering is the process of interconnecting Autonomous Systems to exchange traffic between them. This is typically done through Border Gateway Protocol (BGP), which is the protocol used to exchange routing information between ASes.

When two ASes establish a peering relationship, they agree to exchange traffic with each other based on certain routing policies. These policies define which prefixes (network blocks) are allowed to be advertised and received by each AS.

What's ASN Lookup API

Greip offers an ASN Lookup API that allows you to retrieve information about any Autonomous System using its ASN. The API returns various details about the ASN, including its name, organisation, registry, country, contact details, domain name, status, IPv4 and IPv6 routes, type of ASN (e.g., ISP, business or hosting), total routes, etc.

How to use the ASN Lookup API

To use the ASN Lookup API, you need to send an HTTP GET request to the Greip API endpoint, specifying the ASN number you want to lookup. The API will return a JSON response containing information about the specified ASN.

Here's an example of how to use the ASN Lookup API to retrieve information about the ASN 15169, which belongs to Google:


The API will return a JSON response similar to the following:

"asn": "AS15169",
"name": "GOOGLE",
"country": "US",
"org": "Google LLC",
"phone": "+1-650-253-0000",
"email": "[email protected]",
"domain": "",
"status": "ASSIGN",
"registry": "ARIN",
"created": "2000-03-30",
"type": "business",
"ipv4_prefixes": ["", "", ...],
"ipv6_prefixes": ["2001:4860:4860::8888/32", ...]

Benefits of using the ASN Lookup API

The ASN Lookup API offers several benefits for network engineers, system administrators, and security professionals:

  • It provides a quick and easy way to retrieve information about an Autonomous System using its ASN.
  • It allows you to identify the organisation that owns an ASN, which can be useful for troubleshooting network issues.
  • It helps you to detect potential security threats, such as malicious traffic originating from a suspicious ASN.
  • It allows you to monitor changes to an ASN, such as changes to its IP address blocks, which can help you to stay on top of network changes.

Overall, the ASN Lookup API is a valuable tool for anyone involved in network operations and security, providing a simple and efficient way to retrieve information about an Autonomous System using its ASN.

ASN Lookup API protect your business from fraud

The ASN Lookup API can be a valuable tool for businesses that want to protect themselves against potential security threats and fraud. By using the API to identify suspicious Autonomous Systems that are generating malicious traffic, businesses can take proactive steps to prevent fraud and protect their customers' data.

Fraudsters often use malicious traffic generated from suspicious ASNs to conduct cyber attacks, such as distributed denial-of-service (DDoS) attacks, phishing scams, and malware distribution. By monitoring traffic from these ASNs, businesses can detect and block these attacks before they cause any harm.

For example, a business that operates an e-commerce website could use the ASN Lookup API to monitor traffic originating from different ASNs. If it identifies traffic from a suspicious ASN that is known to be associated with fraud or cyber attacks, the business can block traffic from that ASN or take other steps to prevent fraud.

Using the ASN Lookup API can be an effective fraud prevention step for businesses of all sizes. By staying vigilant and monitoring traffic from suspicious ASNs, businesses can protect themselves and their customers against potential security threats and prevent financial losses due to fraud.

For more information about Greip API, please visit our documentation.


ASNs play a critical role in the operation of the Internet. They allow network traffic to be routed correctly and efficiently, and they facilitate the interconnection of different Autonomous Systems. Understanding ASNs and their significance is essential for network engineers, ISPs, and anyone involved in Internet infrastructure.


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