Every block of public IP addresses is announced to the rest of the internet by an autonomous system, and each autonomous system has a number assigned by a regional internet registry. Google is AS15169, Amazon's main cloud network is AS16509, and every consumer broadband carrier has its own.
Why proxy buyers care
Websites and anti-bot vendors look up the ASN of every incoming IP and classify the network behind it: consumer ISP, mobile carrier, hosting provider, business or education. Traffic from a hosting ASN is easy to label as a server, which is why datacenter proxies are the first to be filtered on strict sites. ISP proxies exist because their addresses are announced by consumer carrier ASNs even though the machines sit in data centres.
How to check an IP's ASN
Team Cymru runs a free whois service that maps an IP to its ASN and owner:
whois -h whois.cymru.com " -v 8.8.8.8"
To check the exit of a proxy, ask an IP information service through it and read the org field, which starts with the AS number:
curl -s -x http://USERNAME:PASSWORD@HOST:PORT https://ipinfo.io/json
Common confusion
An ASN tells you who announces a range, not where a given address physically is or what kind of device uses it. Location comes from geolocation databases, which are separate and often disagree. The long version, with lookup tools and how sites score each ASN type, is in what is an ASN.