Autonomous System Numbers (ASNs) are fundamental building blocks of internet routing. This guide explains what ASNs are, how to look them up, and practical applications for developers and network administrators.
What is an ASN?
An Autonomous System Number (ASN) is a unique identifier assigned to autonomous systems (AS) - collections of IP networks and routers under single administrative control that share a common routing policy.
- Unique 16-bit or 32-bit identifier for routing domains
- Assigned by Regional Internet Registries (RIRs)
- Used in BGP (Border Gateway Protocol) routing
- Identifies the organization controlling IP address blocks
- Essential for understanding internet topology
ASN Ranges and Types
16-bit ASNs (Legacy)
Original 16-bit ASNs range from 1 to 65535:
- 1-64495: Public ASNs for internet routing
- 64496-64511: Documentation and examples
- 64512-65534: Private ASNs for internal use
- 65535: Reserved
32-bit ASNs (Modern)
32-bit ASNs accommodate internet growth:
- 65536-4199999999: Public 32-bit ASNs
- 4200000000-4294967294: Private 32-bit ASNs
- Written as plain numbers or dot notation (65536.0)
How ASN Lookup Works
ASN lookup involves querying databases that map IP addresses to their controlling autonomous systems:
- IP-to-ASN mapping databases (like MaxMind)
- BGP routing table analysis
- WHOIS queries to RIRs
- Real-time BGP feeds and looking glasses
// Example ASN lookup API request
fetch('https://api.whatismyip.io/asn/lookup?ip=8.8.8.8')
.then(response => response.json())
.then(data => {
console.log('ASN:', data.asn);
console.log('Organization:', data.organization);
console.log('Country:', data.country);
});Developer Use Cases
Network Security Analysis
- Identify hosting providers for threat intelligence
- Detect traffic patterns and anomalies
- Block or allow traffic by ASN
- Correlate attacks across IP ranges
Content Delivery Optimization
- Route traffic to optimal CDN nodes
- Implement ASN-based load balancing
- Optimize peering relationships
- Monitor network performance by provider
Compliance and Geo-blocking
- Identify cloud providers for data residency
- Implement region-specific routing policies
- Detect VPN/proxy usage by ASN patterns
- Enforce corporate network policies
API Integration Examples
Common integration patterns for ASN lookup:
// Bulk ASN lookup for log analysis
const ips = ['8.8.8.8', '1.1.1.1', '208.67.222.222'];
const lookupBatch = async (ipList) => {
const results = await Promise.all(
ipList.map(ip =>
fetch(`/api/asn/lookup?ip=${ip}`)
.then(r => r.json())
)
);
return results;
};
// Group IPs by ASN
const groupByASN = (results) => {
return results.reduce((groups, item) => {
const asn = item.asn;
groups[asn] = groups[asn] || [];
groups[asn].push(item);
return groups;
}, {});
};ASN Information Fields
Typical data returned by ASN lookups:
- ASN number: The autonomous system identifier
- Organization name: Company or entity controlling the ASN
- Country: Primary country of registration
- Registry: RIR that allocated the ASN (ARIN, RIPE, etc.)
- IP ranges: Prefixes announced by this ASN
- Type: ISP, hosting, educational, government, etc.
Use our ASN Lookup tool to explore autonomous systems and understand network ownership patterns for any IP address or range.
Practical Applications
Log Analysis and Monitoring
// Analyze web server logs by ASN
const analyzeTrafficByASN = (logEntries) => {
const asnStats = {};
logEntries.forEach(entry => {
const asn = entry.visitor_asn;
asnStats[asn] = asnStats[asn] || {
requests: 0,
bytes: 0,
organization: entry.asn_organization
};
asnStats[asn].requests++;
asnStats[asn].bytes += entry.response_bytes;
});
return asnStats;
};Security Automation
// Firewall rule generation based on ASN
const generateFirewallRules = (blockedASNs) => {
return blockedASNs.map(asn => ({
action: 'block',
source: `asn:${asn}`,
description: `Block traffic from ASN ${asn}`,
priority: 100
}));
};
// Example: Block known hosting ASNs for user registration
const hostingASNs = [16509, 14618, 13335]; // AWS, Amazon, Cloudflare
const rules = generateFirewallRules(hostingASNs);Major ASN Examples
Common ASNs you might encounter:
- AS15169: Google LLC (Google services, YouTube)
- AS16509: Amazon.com (AWS infrastructure)
- AS13335: Cloudflare (CDN and security services)
- AS32934: Facebook/Meta (social media platforms)
- AS8075: Microsoft Corporation (Azure, Office 365)
- AS7018: AT&T Services (major US ISP)
- AS174: Cogent Communications (Tier 1 provider)
Best Practices
- Cache ASN lookup results to reduce API calls
- Update ASN databases regularly as allocations change
- Consider both IPv4 and IPv6 ASN mappings
- Implement fallback mechanisms for lookup failures
- Monitor ASN changes for critical infrastructure
- Use ASN data alongside other network intelligence
Conclusion
ASN lookup is a powerful tool for understanding network topology, implementing security policies, and optimizing network performance. Whether you're analyzing traffic patterns, implementing geo-blocking, or investigating security incidents, ASN data provides valuable context about network ownership and routing.