When you look up an IP, alongside the location and ISP you may have seen a number beginning with "AS", such as AS15169. That is the ASN. Many people skip past it, but when it comes to determining who an IP really belongs to and what type it is, the ASN is often more reliable than the location.
The earlier articles kept noting that network type is the heaviest factor in a risk score, and the ASN is the most direct basis for determining network type. This article makes it clear.
What an ASN is
An ASN (Autonomous System Number) identifies an "autonomous system" on the internet — a set of IP addresses managed by a single organisation under one routing policy.
An analogy: if the whole internet were the global postal network, each ASN would be an independently run postal region. It has its own number, its own range of addresses to handle, and its own rules for interconnecting with other regions. Getting data from one place to another is a relay between these regions.
ASN holders are typically: network operators (China Telecom, Comcast, Vodafone), cloud and hosting providers (AWS, Google, Cloudflare), and large organisations (universities, multinationals). One organisation can hold multiple ASNs, and one ASN can manage many ranges of IPs.
What an ASN looks like
An ASN is written as "AS" followed by a number:
AS15169 → Google LLC
AS13335 → Cloudflare
AS4134 → China Telecom backbone
AS16509 → Amazon AWSThe numbers are globally unique, assigned by Regional Internet Registries (RIRs). Early ASNs were 16-bit (up to 65535); once that ran short they were extended to 32-bit, which is why you now see some very large numbers.
How ASN, IP and ISP relate
These three are easy to confuse, so to lay them out:
- IP address: the address of a specific device, e.g. 8.8.8.8
- ASN: the number of the "autonomous system" managing a batch of IPs
- ISP / organisation: the entity that actually holds and operates that ASN
The hierarchy is: an ISP holds one or more ASNs, and each ASN manages several ranges of IPs.
So starting from an IP and finding which ASN it belongs to tells you which organisation manages it and what kind of network it is. That chain is the core of determining IP ownership.
Why the ASN is more reliable than location
The most visible information when looking up an IP is its location (country, city), but for judging "what kind of address this is", location is actually less reliable than the ASN. Two reasons:
Location is geographic; the ASN is ownership. Location tells you roughly where an IP is, not who owns it or what it is used for. An address might be geographically in the US yet belong to a cloud host — and that "belongs to whom" is what decides whether it is a datacenter or residential IP.
Location databases can be influenced; ASN ownership is hard to disguise. The article on datacenter versus residential IPs mentioned "broadcast IPs" — using route announcements to make databases believe an IP is somewhere it is not. Location can be swayed that way, but which ASN actually announces a range, and who that ASN belongs to, is written into the global routing table and cannot be faked.
So when a risk system determines network type, it looks mainly at which category the ASN falls into, not which country the location shows.
How to look up an IP's ASN
The fastest way: open the IP lookup, enter any IP, and the result shows its ASN and holding organisation directly. Leave it blank to check your own IP; the page detects it automatically.
Command line: if you prefer the terminal, you can query with whois:
whois -h whois.cymru.com " -v 8.8.8.8"Or check the OriginAS field in the IP's whois record.
Reverse lookup from an ASN: given an ASN, you can also look up which IP ranges it manages and which networks it peers with. Such information is available in the RIRs' public databases (ARIN, RIPE, APNIC and others).
What ASN information reveals
With an IP's ASN in hand, you can infer several useful things:
Network type. If the ASN's holder is a consumer ISP, it is likely a residential IP; if it is a cloud or hosting provider, it is a datacenter IP. This is the most valuable inference.
Common origin. If two IPs belong to the same ASN, they are managed by the same organisation. This helps identify addresses that look scattered but share an origin — a batch of IPs spread across cities that all belong to one niche host's ASN have the same underlying nature.
Scale and reputation. A large operator's ASN manages a vast pool of addresses serving real users and has a solid reputation baseline, whereas some niche ASNs dedicated to proxies or VPSes may carry a lower overall reputation.
A common misconception
Do not treat the size of an ASN number as a sign of age or quality. The number is just allocation order. AS15169 (Google) is smaller than some five-digit niche host's ASN merely because it was allocated earlier; it signals nothing about superiority. To judge nature, look at who the holder is, not the size of the number.
Another often-overlooked point: ASN ownership changes too. A range of IPs or an ASN can be resold, merged or reassigned. Risk databases often lag months behind such changes, which is why classifications are occasionally inaccurate — one of the reasons behind datacenter/residential misclassification.
Summary
An ASN is the number identifying "who manages a batch of IPs". It matters because it answers the question of ownership, and ownership determines network type — the heaviest factor in a risk judgement.
Compared with the easily-influenced location, the ASN is a more reliable basis for judgement. Next time you look up an IP, do not just note which country it is in — glance at who its ASN belongs to, which usually says more about the address's true nature. For a quick look, use the IP lookup.