What Is an ASN? Autonomous System Number Explained

An ASN (Autonomous System Number) is a unique identifier assigned to a network or group of IP networks that operates under a single routing policy. Understanding what an ASN is and how it works is critical for proxy users, network engineers, and anyone involved in web scraping or data collection at scale.

Every major ISP, cloud provider, and large organization has at least one ASN. When you use a proxy, the ASN associated with that proxy’s IP address reveals which network it belongs to — and websites increasingly use ASN data to detect and block proxy traffic.

How Autonomous System Numbers Work

The internet is composed of thousands of interconnected autonomous systems (AS). Each AS is a network or collection of networks managed by a single organization that presents a unified routing policy to the internet.

The Role of ASNs in Internet Routing

ASNs serve as the backbone of internet routing through the Border Gateway Protocol (BGP):

1. Identification: Each AS receives a unique ASN from a Regional Internet Registry (RIR)
2. Routing: ASNs announce their IP prefixes to neighboring networks via BGP
3. Path Selection: Routers use ASN information to determine the best path for data packets
4. Policy Enforcement: Network operators use ASNs to implement routing policies and traffic agreements

ASN Format

ASNs come in two formats:

The 32-bit format was introduced in 2007 as the original 16-bit pool neared exhaustion.

Who Assigns ASNs?

Five Regional Internet Registries (RIRs) manage ASN allocation:

• ARIN — North America
• RIPE NCC — Europe, Middle East, Central Asia
• APNIC — Asia Pacific
• LACNIC — Latin America, Caribbean
• AFRINIC — Africa

Organizations must demonstrate a need for a unique routing policy to receive an ASN assignment.

ASNs and Proxy Services

ASN information is highly relevant to proxy usage because it reveals the true nature of an IP address’s network origin.

How Websites Use ASN Data

Websites and anti-bot systems check ASN data to:

• Identify datacenter IPs: ASNs belonging to cloud providers (AWS, Google Cloud, DigitalOcean) indicate datacenter proxies
• Verify residential IPs: ASNs from ISPs (Comcast, AT&T, BT) suggest legitimate residential connections
• Detect proxy networks: Known proxy provider ASNs may be flagged or blocked
• Enforce geo-restrictions: ASN data confirms geographic location beyond just IP geolocation

ASN Categories for Proxy Users

ASN Targeting in Proxy Services

Premium proxy providers offer ASN-level targeting, allowing you to:

• Select proxies from specific ISPs (e.g., only Comcast residential IPs)
• Avoid ASNs known to be blocked by target websites
• Match the ASN profile expected for a particular geography
• Diversify traffic across multiple ASNs to avoid pattern detection

# Example: Requesting a proxy from a specific ASN
import requests

proxy_url = "http://user:pass@gate.provider.com:7777"
Some providers support ASN targeting via username parameters
proxy_url = "http://user-asn-7922:pass@gate.provider.com:7777"

proxies = {
"http": proxy_url,
"https": proxy_url
}

response = requests.get("https://httpbin.org/ip", proxies=proxies)
print(response.json())

How to Look Up an ASN

Online ASN Lookup Tools

Several tools let you look up ASN information:

1. IPinfo.io — https://ipinfo.io/AS13335
2. BGPView — https://bgpview.io/asn/13335
3. RIPE Database — https://stat.ripe.net/AS13335
4. PeeringDB — https://www.peeringdb.com/asn/13335

Command-Line ASN Lookup

# Using whois
whois -h whois.radb.net AS13335

Using dig for IP-to-ASN mapping
dig +short TXT 8.8.8.8.origin.asn.cymru.com
Returns: "15169 | 8.8.8.0/24 | US | arin | 2000-03-30"

Using curl with IPinfo
curl https://ipinfo.io/8.8.8.8/org
Returns: AS15169 Google LLC

Python ASN Lookup

import ipwhois

Look up ASN for an IP address
obj = ipwhois.IPWhois('8.8.8.8')
result = obj.lookup_rdap()

print(f"ASN: {result['asn']}")
print(f"ASN Description: {result['asn_description']}")
print(f"ASN Country: {result['asn_country_code']}")

Notable ASNs Every Proxy User Should Know

Major Cloud/Datacenter ASNs (High Detection Risk)

Major Residential ISP ASNs (Low Detection Risk)

ASN Diversity in Proxy Strategy

Why ASN Diversity Matters

When scraping at scale, requests from a single ASN create patterns that trigger detection:

• Concentration risk: Too many requests from one ASN signals automated activity
• Bulk blocking: Websites may block entire ASN ranges
• Behavioral profiling: Same-ASN traffic with identical patterns is easily flagged

Best Practices for ASN Management

1. Distribute across ASNs: Spread requests across multiple residential ASNs
2. Match expected ASNs: Use ASNs typical for the target website’s audience geography
3. Avoid known proxy ASNs: Some ASNs are widely flagged by anti-bot systems
4. Monitor ASN reputation: Track which ASNs are being blocked by your target sites
5. Mix ASN types: Combine residential and mobile carrier ASNs for diverse traffic profiles

ASN vs. IP Subnet: Understanding the Difference

Websites may block at the ASN level (blocking all IPs from a datacenter) or subnet level (blocking a specific /24 range), depending on their security posture.

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Frequently Asked Questions

Can I get my own ASN?

Yes, but it requires justification. You must demonstrate a multi-homed network (connected to at least two ISPs) and apply through your Regional Internet Registry. ASN assignments cost approximately $500-$1,000 annually.

How do websites detect datacenter ASNs?

Websites maintain databases mapping ASNs to their organization type. Services like MaxMind, IPinfo, and IP2Location classify ASNs as residential, datacenter, or mobile. If your proxy’s IP belongs to a datacenter ASN like AWS (AS16509), it’s immediately flagged.

Do residential proxies always use residential ASNs?

Quality residential proxies should use genuine residential ISP ASNs. However, some low-quality providers route traffic through datacenter ASNs or use misclassified IPs. Always verify your proxy’s ASN using lookup tools before relying on it for sensitive tasks.

What is ASN-level blocking?

ASN-level blocking occurs when a website blocks all IP addresses belonging to a specific autonomous system. For example, blocking AS14061 (DigitalOcean) would prevent any DigitalOcean-hosted proxy from accessing the site. This is more aggressive than IP-level blocking.

How many ASNs exist worldwide?

As of 2026, there are approximately 120,000+ allocated ASNs globally, with roughly 80,000 actively routing traffic. The number grows as more organizations require unique routing policies.