What Is an ASN? The Number That Decides If You Get Blocked

People notice that datacenter proxies get blocked and residential ones do not, and usually assume it is something subtle. It is not. It is a number attached to your IP address that anyone can look up in milliseconds: the ASN. Understanding it explains most of what happens at the network layer of bot detection — and why no amount of header tuning changes the outcome.

What an ASN Actually Is

An Autonomous System Number identifies a network operator that controls a block of IP addresses and announces routes for them on the public internet. Every routable IP belongs to exactly one, and the mapping is public by design — the internet could not route traffic otherwise.

Broadly they fall into two camps:

• Consumer ISPs — the networks serving homes and phones. Their addresses are handed to ordinary people on broadband and mobile connections.
• Hosting and cloud providers — networks serving servers. Their addresses belong to machines in data centres.

That distinction is the whole story. A website checking your ASN is asking one question: is there plausibly a human on the end of this connection?

Looking It Up

You can check any address instantly — and so can every site you visit:

# command line
whois 8.8.8.8 | grep -i "origin\|orgname"

# or in Python against a public lookup service
import httpx
r = httpx.get("https://ipapi.is/json/?q=8.8.8.8", timeout=10)
d = r.json()
print(d.get("asn", {}).get("asn"), d.get("asn", {}).get("org"),
      "| type:", d.get("asn", {}).get("type"))

The type field is the one that matters — typically something like isp, hosting, or business. You can check what your own proxy exits as with our free IP lookup tool.

Why It Is Checked First

Of all the signals in modern bot detection, ASN is the cheapest to evaluate. No JavaScript, no fingerprinting, no behavioural analysis — just a lookup against a table the site already has. It happens before your request is even parsed, which is why:

• Datacenter IPs are often blocked instantly, before any of your careful headers are read.
• Whole ranges go down together. Blocking is applied at ASN or subnet level, so one abusive tenant on a cloud provider can get neighbours blocked too.
• It is deterministic. Behavioural detection is probabilistic; ASN classification is a fact lookup.

Anti-bot vendors maintain reputation scores per ASN and per subnet, informed by traffic across every site they protect. An address can therefore arrive somewhere new with a reputation already attached — not because of anything you did.

The Part You Cannot Fake

This is where people waste weeks. Your user agent, headers, TLS fingerprint and behaviour are all things your client controls. Your ASN is not. It is a property of the address you are connecting from, determined by whoever owns and routes that block.

You cannot spoof it, override it in a header, or hide it behind a browser. The only way to change your ASN is to genuinely exit from a different network. That is precisely what a residential proxy does: your request leaves from an address belonging to a real consumer ISP, so the lookup returns a consumer ISP — because it is one.

How This Maps to Proxy Types

Mobile deserves a note. Carriers place many real subscribers behind shared addresses, so blocking one is expensive for a site — it takes real customers with it. That reluctance is why mobile IPs survive on the strictest platforms. See datacenter vs residential.

What To Do With This

• Check what you are actually exiting as. Not what the provider claims — look it up. If the type says hosting, expect hosting treatment.
• Match the ASN to the job. Protected consumer sites want a consumer ISP. An unprotected API does not care, so pay datacenter prices there.
• Do not fight it with headers. If you are blocked at the network layer, no user-agent change will help. Change the network.
• Treat cheap pools with suspicion. Unusually cheap "residential" that resolves to a hosting ASN is datacenter with better marketing.
• Remember reputation is shared. On a poorly-run pool you inherit other users' behaviour, which is one reason how a provider sources its IPs affects your success rate, not just your conscience.

Frequently Asked Questions

What is an ASN?

An Autonomous System Number identifies the network operator that controls a block of IP addresses and announces routes for them. Every routable IP belongs to one, and the mapping is public. Websites use it to tell whether an address belongs to a consumer ISP or to a hosting provider.

Why do websites block datacenter ASNs?

Because an address registered to a hosting provider almost certainly belongs to a server, not a person browsing. Checking the ASN is instant and requires no JavaScript or fingerprinting, so it is the cheapest possible filter and is applied before anything else about the request is examined.

Can you change or hide your ASN?

No. Unlike your user agent, headers or browser fingerprint, the ASN is a property of the address you connect from and is determined by whoever owns and routes that block. The only way to change it is to genuinely exit from a different network, which is what a residential proxy does.

How do I check the ASN of my proxy?

Make a request through the proxy to an IP information service and read the ASN and its type. If the type reports hosting rather than ISP, the address will be treated as a datacenter IP regardless of how it was sold to you.

Conclusion

The ASN is the first thing checked and the one thing you cannot spoof. It is why datacenter proxies fail on protected sites before your headers are even read, why blocks land on whole ranges at once, and why residential and mobile IPs keep working — they belong to networks full of real people. Check what your traffic actually exits as, match the network type to the target, and stop trying to solve a network-layer problem at the application layer.

Exit from a real consumer ISP: SpyderProxy residential proxies from $2.75/GB — genuine ISP-assigned addresses, ethically sourced, across 195+ countries.