ASN (autonomous system number)

An ASN, or autonomous system number, is a unique number assigned to a network that runs its own routing policy on the public internet — and every IP address in use belongs to one. The numbers are handed out by the regional internet registries, and the mapping from address to ASN is public.

Why it matters when you buy proxies

The ASN is the first thing an anti-fraud system checks about your IP, and it is a lookup, not a guess. It says who operates the network: a mobile carrier, a consumer broadband provider, or a hosting company. That single field is what separates a mobile address from a datacenter proxy in a site's eyes, before any of your traffic is scored.

This is also why cheap proxies fail in ways you cannot fix by behaving well. If the ASN behind your address is a known hosting provider, no amount of careful pacing changes the category you were placed in on the first request.

A concrete example

Paste any IP into a public ASN lookup and you get back the operator's name and the address block it sits in. A site can run that same lookup on you. If it returns a mobile carrier, you are in the same bucket as every phone on that network; if it returns a cloud host, you are in the bucket that gets rate-limited first and considered as part of the IP reputation score.