Why ASN Blocking Belongs in Your Security Playbook

In the modern threat landscape, organizations are bombarded by malicious IP addresses on a daily basis. From brute-force login attempts to botnet-driven crawlers, defenders are often left playing whack-a-mole, blocking one IP only to be hit by dozens more from the same source. This approach doesn’t scale, and it burns valuable time when speed matters most.

That’s where ASN blocking comes in.

What is an ASN?

Every network on the internet belongs to an Autonomous System Number (ASN). Think of an ASN as an ISP’s “license plate” on the internet, it identifies the organization that owns and manages a block of IP ranges.

• AS15169 → Google

• AS13335 → Cloudflare

• AS58224 → Iran Telecom

Instead of blocking a single IP address, you can block the entire ASN. This means every IP within that network’s ownership is denied access at once.

Why Attackers Cluster in ASNs

Cybercriminals often take advantage of...

• Shady hosting providers that turn a blind eye to abuse.

• Cheap VPS services that enable disposable botnet nodes.

• Compromised ISPs with large swaths of vulnerable endpoints.

As a result, malicious traffic frequently originates from just a handful of ASNs. Blocking the ASN directly can be far more effective than chasing thousands of individual IPs.

The Power of ASN Blocking

When used correctly, ASN blocking offers...

• Efficiency: A single rule can eliminate thousands of malicious IPs.

• Speed: Rapid containment during brute force or scanning campaigns.

• Scale: Ideal for cutting down attack noise during large botnet events.

The Risks--> A Blunt but Effective Tool

ASN blocking is powerful, but it is not without risks. Some ASNs are enormous... Amazon AWS, Microsoft Azure, and Cloudflare host a mix of both good and bad traffic. Blocking them outright could inadvertently shut out legitimate users or services.

Additionally, ASN ownership can change, and IP ranges are dynamic. A block that makes sense today may cause problems tomorrow if not reviewed regularly.

How to Implement ASN Blocking

There are several ways to bring ASN blocking into your defenses:

• Enterprise Firewalls (Fortinet, Palo Alto, Cisco, Sophos): These often include ASN or Internet Service Database (ISDB) objects you can apply directly in firewall rules.

• Linux Firewalls (iptables + ipset): Scripts can pull IP ranges for a given ASN from routing databases (RADb, Team Cymru) and load them into blocklists.

• Cloud WAFs (Cloudflare, Akamai, AWS WAF): Many offer ASN-based filtering with a simple rule selection.

ASN blocking won’t replace traditional defenses like IP reputation lists or intrusion prevention systems. Instead, it should be viewed as a strategic tool in the defender’s playbook, one that can deliver fast, broad impact when malicious traffic is clearly tied to a specific network.

When IP blocking feels like playing whack-a-mole, ASN blocking is more like shutting down the entire arcade machine.