ASN (Autonomous System Number)

An Autonomous System Number (ASN) is a unique number assigned to a network that controls its own block of IP addresses and decides how traffic reaches them. Comcast has one. Amazon Web Services has several. Vodafone has its own. Together, these networks are the pieces the internet is assembled from.

Think of IP addresses as street addresses and ASNs as the towns they sit in. Knowing the town tells you a great deal about the neighbourhood, even before you look at the individual house.

What it is actually for

ASNs exist so networks can route traffic between each other. Using a protocol called BGP, each network announces to its neighbours which IP ranges it is responsible for. Those announcements let any packet find a path from one side of the world to the other. Without ASNs there would be no way to work out where anything belongs.

Why it matters so much for proxies

Here is the part people miss. Every IP address maps to an ASN, and that lookup is instant and free. Anti-bot systems do it on every request, because the ASN answers a question the IP alone cannot: what kind of connection is this?

• An ASN belonging to Comcast, BT, or Deutsche Telekom means a residential customer. Trusted.
• An ASN belonging to AWS, DigitalOcean, or Hetzner means a server in a data centre. Immediately suspicious for consumer-facing traffic, because ordinary shoppers do not browse from cloud hosts.
• An ASN belonging to Vodafone or T-Mobile means a mobile carrier. Highly trusted, thanks to shared carrier addressing.

This is precisely why datacenter proxies get blocked on strict sites no matter how clean their headers are. You can strip every revealing header and still be identified in one lookup, because the address itself is registered to a hosting company. It is also why residential and ISP proxies command a premium — you are paying for an address that sits under a trustworthy ASN.

How sites use it in practice

Beyond simple classification, ASN data drives fraud scoring, geo-restriction enforcement, rate limits applied per network rather than per IP, and blanket blocks on hosting providers known for abuse. Some sites also flag a mismatch, such as a checkout claiming to be in one country from an ASN that operates entirely in another.

Checking one

Any IP lookup tool will show the ASN and owning organisation for an address. It is genuinely worth doing before committing to a proxy provider: run a sample of their IPs through a lookup and confirm the ASNs really are consumer ISPs rather than hosting companies. Networks advertised as residential sometimes turn out not to be.