You ran an IP lookup, saw a field labeled ASN with a value like AS15169, and wondered what it actually means. An ASN, or Autonomous System Number, is the unique ID for the network that owns and routes that IP address. This guide explains what an ASN is, how ASNs work with BGP, the difference between public and private ASNs, how they are assigned, and how to look one up.
What is an ASN (Autonomous System Number)?
An Autonomous System Number (ASN) is a globally unique number that identifies an autonomous system: a network, or group of IP ranges, run under a single routing policy by one organization. ASNs let independent networks exchange routing information across the internet. Each ASN is written with an AS prefix, such as AS15169 for Google.
Think of the internet as a network of networks. Each one needs a stable, unique name so the others know how to reach it, and the ASN is that name. Individual IP addresses come and go, but the ASN identifies the operator responsible for a whole block of addresses.
What is an autonomous system?
An autonomous system (AS) is a collection of connected IP address ranges, called prefixes, that operate under one clearly defined routing policy and administration. An internet service provider, a cloud platform, or a large enterprise each runs its own autonomous system. The ASN is the identifier that represents that system to every other network on the internet.
Comcast, Amazon Web Services, and Cloudflare each operate one or more autonomous systems. When you load a website, your request usually crosses several of them before it reaches the server, and each handoff is decided using ASNs.
Why do ASNs exist? ASNs and BGP routing
ASNs exist so independent networks can route traffic between each other. The Border Gateway Protocol (BGP) uses ASNs to announce which IP ranges each network can reach. Routers collect those announcements into a global routing table, then use it to pick the best path from one autonomous system to the next.
Every AS effectively tells its neighbors, "these prefixes are reachable through me." Those announcements propagate across the internet until every BGP router knows which ASN to hand a packet to next. Without ASNs, there would be no consistent way to identify who announces a route, and global routing would not scale.
2-byte vs 4-byte ASNs (ASN format)
ASNs come in two sizes. The original 2-byte (16-bit) format runs from 0 to 65,535, about 65,000 numbers. To meet demand, the 4-byte (32-bit) format was added, running from 0 to 4,294,967,295, over 4 billion numbers. Both are written the same way: AS followed by the number.
The move to 4-byte ASNs (defined in RFC 6793) happened because the 2-byte pool was running low. For backward compatibility, older routers show any 4-byte ASN they cannot interpret as AS23456, a placeholder called AS_TRANS. You may also see 4-byte ASNs written in asdot notation, such as 1.10, instead of a single asplain number.
Public vs private ASNs
Public ASNs are globally unique and used to route traffic across the public internet. Private ASNs are reserved for internal use and never appear in the global routing table. RFC 6996 reserves 64512 to 65534 for 16-bit private ASNs and 4200000000 to 4294967294 for 32-bit private ASNs.
Private ASNs suit large organizations that run BGP internally but do not need a globally unique identity for every segment. A few numbers are reserved outright: AS0 is reserved, and AS65535 and AS4294967295 are set aside and should not be used in routing.
- 0 to 65,535: the original 2-byte (16-bit) ASN range.
- 64,512 to 65,534: private use within the 2-byte range (RFC 6996).
- 0 to 4,294,967,295: the expanded 4-byte (32-bit) ASN range.
- 4,200,000,000 to 4,294,967,294: private use within the 4-byte range (RFC 6996).
How are ASNs assigned?
ASNs are handed out through a tiered system. IANA allocates blocks of ASNs to the five Regional Internet Registries, ARIN, RIPE NCC, APNIC, LACNIC, and AFRINIC, which then assign individual ASNs to network operators that meet their criteria. Most organizations request an ASN through their regional registry or their upstream provider.
You generally need your own ASN when you run a multi-homed network, meaning you connect to more than one provider and want to control your own routing. A single-homed network usually does not need one.
How many ASNs are there?
About 120,000 ASNs are currently assigned to network operators worldwide, out of more than 400,000 that IANA has delegated to the regional registries. That is still a tiny share of the 4-byte space, which holds over 4 billion possible ASNs, so the supply is nowhere near running out.
ASN vs IP address vs CIDR block
These three describe the internet at different zoom levels. An IP address identifies a single endpoint. A CIDR block, or prefix, is a range of IP addresses such as 8.8.8.0/24. An ASN sits one level up: it identifies the network that owns and announces one or more CIDR blocks.
One ASN can announce many prefixes, and each prefix contains many IP addresses. When you look up an IP, the ASN tells you which network operator stands behind it.
- IP address: one device or endpoint, for example 8.8.8.8.
- CIDR block or prefix: a range of IPs, for example 8.8.8.0/24.
- ASN: the network that owns and routes those prefixes, for example AS15169.
How to find the ASN of an IP address
To find the ASN behind an IP address, enter the IP into an ASN lookup tool. It returns the ASN, the organization that operates it, the ASN type (ISP, hosting, business, education, or government), and the operator's domain. You can look up any ASN with Abstract's free ASN Lookup tool, no signup required.
An ASN lookup answers a different question than WHOIS. WHOIS tells you who registered a domain or IP block; an ASN lookup tells you which network is currently routing that IP, which is what matters for traffic analysis and risk scoring.
Why ASNs matter for fraud detection and security
An IP's ASN tells you who really operates it, which is a strong signal for fraud and abuse. Traffic from a hosting or data-center ASN often points to a bot, VPN, or proxy rather than a real residential user, so knowing the ASN and its type lets you treat those requests differently.
This is where an ASN becomes a risk signal rather than trivia. Abstract's IP Intelligence API returns the ASN, its type, and flags such as is_hosting, is_vpn, is_proxy, and is_tor for any IP address, so you can score risk in real time at signup, checkout, or login. For location tied to the same IP, the IP Geolocation API adds country, region, and city.
Examples of well-known ASNs
Large operators are easy to recognize once you know their ASNs. A few well-known examples:
- AS15169: Google
- AS13335: Cloudflare
- AS16509: Amazon (AWS)
- AS8075: Microsoft
- AS32934: Meta (Facebook)
When an IP lookup returns one of these ASNs, you know the request is coming from that company's network, often a cloud or data-center environment rather than a home connection.
Frequently Asked Questions
What does ASN stand for?
ASN stands for Autonomous System Number. It is a globally unique number that identifies an autonomous system, the network or group of IP ranges that one organization operates under a single routing policy.
What is an ASN used for?
An ASN is used to route traffic between networks on the internet. The Border Gateway Protocol (BGP) uses ASNs to announce which IP ranges each network can reach, so routers can choose a path from one autonomous system to the next.
What is the difference between a public and a private ASN?
A public ASN is globally unique and used to route traffic across the public internet. A private ASN is for internal use only and never appears in the global routing table. RFC 6996 reserves 64512 to 65534 and 4200000000 to 4294967294 for private use.
What is my ASN?
Your ASN is the identifier of the network operating your current IP address, usually your internet provider or hosting company. Enter your IP into Abstract's free ASN Lookup tool to see the ASN, the operator's name, and its type.
Is an ASN the same as an IP address?
No. An IP address identifies a single device or endpoint, while an ASN identifies the whole network that owns and routes a range of IP addresses. One ASN can be responsible for thousands or millions of IP addresses.
What is a 4-byte ASN?
A 4-byte ASN is a 32-bit Autonomous System Number, ranging from 0 to 4,294,967,295. It was introduced because the original 2-byte (16-bit) range of 0 to 65,535 was running out. Both formats are written as AS followed by the number.
