ASN Lookups: Understanding Autonomous System Numbers

An Autonomous System Number (ASN) is a unique identifier assigned to a network that controls a set of IP prefixes on the internet. Every ISP, cloud provider, university, and large enterprise typically has one or more ASNs.

What you can learn from an ASN

• Owner name and domain — Google's ASN 15169 tells you immediately this is Google infrastructure.
• Network type — Residential ISP vs. datacenter vs. mobile carrier vs. government.
• Geographic scope — Some ASNs are country-specific; others (like AWS) are global.
• Abuse history — Some ASNs are associated with high volumes of spam or attack traffic.

ASNs for access control

Rather than blocking individual IPs, you can block or allow entire ASNs. Blocking AWS, Azure, and GCP ASNs prevents most cloud-based scraping. Allowing your corporate ASN can replace VPN requirements for internal tools.

Querying ASN data

GET https://api.openipapi.com/v1/asn/15169
X-API-Key: your_key

{
  "asn": 15169,
  "name": "Google LLC",
  "domain": "google.com",
  "country": "US",
  "prefixes_v4": 1247,
  "prefixes_v6": 351
}