BGP AS Lookup

Understanding BGP and Autonomous Systems

What is an Autonomous System?

An Autonomous System (AS) is a collection of connected IP routing prefixes under the control of one or more network operators that presents a single, clearly defined routing policy to the internet. Large organizations like ISPs, cloud providers, and enterprises operate their own AS.

AS Numbers (ASN)

Each AS is assigned a unique AS Number (ASN) by a Regional Internet Registry (RIR). Originally 16-bit numbers (0-65535), ASNs were expanded to 32-bit (up to 4294967295) to accommodate internet growth. ASNs are formatted as "AS" followed by the number (e.g., AS15169).

Border Gateway Protocol (BGP)

BGP is the routing protocol that powers the internet. It enables Autonomous Systems to exchange reachability information and determine the best paths for traffic. BGP routing decisions consider policies, AS path length, and various attributes to optimize connectivity.

IP Prefixes

An IP prefix is a block of IP addresses represented in CIDR notation (e.g., 192.0.2.0/24). Autonomous Systems announce their prefixes via BGP to tell other networks how to reach them. The number of prefixes indicates the size and complexity of an organizations network.

Regional Internet Registries (RIRs)

Regional Internet Registries (RIRs) are responsible for allocating IP addresses and AS numbers in their respective regions. There are five RIRs that cover the entire world:

ARIN

North America, parts of Caribbean

RIPE NCC

Europe, Middle East, Central Asia

APNIC

Asia Pacific region

LACNIC

Latin America, Caribbean

AFRINIC

Africa

Use Cases for BGP AS Lookup

• Network Troubleshooting: Identify which organization controls specific IP ranges to diagnose connectivity issues or routing problems.

• Security Research: Investigate suspicious IP addresses to understand which network they originate from and contact the appropriate abuse team.

• Peering Analysis: Research potential peering partners by examining their prefix announcements, geographic presence, and network size.

• DDoS Mitigation: During attacks, identify the source AS to coordinate with upstream providers or implement targeted filtering.

• Compliance and Due Diligence: Verify network ownership for vendor assessments, compliance audits, or business partnerships.

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.