BGP AS Lookup

What Is BGP AS Lookup

BGP (Border Gateway Protocol) is the routing protocol that holds the Internet together. It enables autonomous systems (AS) — independently operated networks such as ISPs, cloud providers, enterprises, and content delivery networks — to exchange routing information and establish paths for traffic to flow between them. Every organization with its own IP address space and Internet connectivity is assigned an Autonomous System Number (ASN).

AS lookup retrieves information about an autonomous system: its assigned IP prefixes, peering relationships, geographic presence, and organizational ownership. This data is essential for network engineering, threat intelligence, incident response, and Internet research.

What an AS Lookup Reveals

Common Use Cases

• Threat intelligence: Identify the network hosting malicious infrastructure by looking up the ASN for attacker IP addresses. Some ASNs are known for hosting bulletproof services.
• Network troubleshooting: Diagnose routing issues by understanding the AS path between your network and a destination, identifying where traffic may be dropping or taking suboptimal routes
• DDoS mitigation: Identify the source ASNs of attack traffic to coordinate mitigation with upstream providers or implement AS-path filtering
• Peering decisions: Evaluate potential peering partners by analyzing their prefix announcements, upstream connectivity, and IXP presence
• IP reputation assessment: Determine the reputation and legitimacy of IP addresses by examining their AS registration, age, and historical behavior

Best Practices

1. Cross-reference multiple data sources — AS information comes from WHOIS, IRR databases, BGP routing tables, and PeeringDB. Cross-reference for accuracy since individual sources may be outdated.
2. Track ASN changes over time — AS assignments and prefix announcements change. Monitor BGP routing changes for your own prefixes and critical upstream providers using tools like BGPStream or RIPE RIS.
3. Use AS data for network security policies — Implement AS-path filtering to accept only expected routes from peers and providers. This prevents BGP hijacking and route leaks.
4. Understand RIR jurisdictions — ARIN (North America), RIPE NCC (Europe/Middle East), APNIC (Asia-Pacific), LACNIC (Latin America), and AFRINIC (Africa) each manage AS and IP allocations in their regions.
5. Investigate suspicious ASNs — ASNs with high concentrations of malicious traffic, frequent prefix hijacking, or bulletproof hosting reputations should be flagged in your threat intelligence processes.