> For the complete documentation index, see [llms.txt](https://docs.ipcheck.ing/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.ipcheck.ing/knowledge-base/concepts/what-is-an-asn.md). # What Is an ASN? Your IP card shows something like `AS15169 Google LLC` or `AS3320 Deutsche Telekom`. What is that number, and why does it appear next to your address? It is the identity of the network your connection belongs to. ## The internet is not one network The word "internet" is short for *inter-network*: a network of networks. There is no single company that owns it and no central switchboard. Instead there are tens of thousands of independently operated networks. Each one is run by a different organisation — an internet provider, a mobile carrier, a university, a hosting company, a large corporation, a government department. Each decides for itself how traffic flows inside it and who it exchanges traffic with. Each of these independent networks is called an **Autonomous System**, or AS. "Autonomous" because it makes its own routing decisions. To identify them, every AS gets a number: an **Autonomous System Number**, or **ASN**. It is written with an `AS` prefix — `AS15169`, `AS13335`, `AS4134`. The numbers are handed out by the same regional registries that hand out IP addresses, so each one is globally unique. An AS also holds blocks of IP addresses, called **prefixes**, written like `203.0.113.0/24`. The AS *announces* its prefixes to its neighbours, which is how the rest of the internet learns "traffic for these addresses should come this way". ## How networks connect: peering and transit Two networks that want to exchange traffic have essentially two options. **Transit** is a customer relationship. A smaller network pays a larger one to carry its traffic to everywhere else on the internet. The larger network says, in effect, "give me your traffic and I will make sure it reaches any destination." Money flows from the smaller network to the larger one. **Peering** is a mutual arrangement. Two networks connect directly and exchange traffic *between their own customers only* — not to the rest of the internet. If a video service and a home broadband provider peer directly, video traffic goes straight across that link instead of taking a longer paid path. Peering is usually settlement-free, because both sides benefit roughly equally. Much of it happens at shared facilities called internet exchange points. | | Transit | Peering | | ------------ | ------------------------ | ------------------------------ | | Relationship | Customer and supplier | Equals | | Reaches | The entire internet | Only each other's own networks | | Payment | Usually paid | Usually free | | Motivation | Need to reach everywhere | Shorter path, lower cost | ```mermaid flowchart TD T1A["Tier 1 network A"] <-->|"peering"| T1B["Tier 1 network B"] T1A -->|"transit (paid)"| R1["Regional provider"] T1B -->|"transit (paid)"| R2["Regional provider"] R1 -->|"transit (paid)"| H["Your home ISP"] R1 <-->|"peering"| R2 H <-->|"peering"| V["Video service"] ``` The path your data takes is the outcome of many such business relationships, negotiated between many independent operators. This is why traffic between two cities in the same country sometimes travels through a third country — the route follows commercial agreements, not geography. ## Tier 1 networks in one paragraph At the top of this structure sits a small group of very large networks, usually called **Tier 1**. The definition is simple: a Tier 1 network reaches every part of the internet without paying anyone for transit. It gets there entirely through peering with the other Tier 1 networks and by carrying its own customers' traffic. Everyone else, no matter how large, buys transit from someone. There are only a handful of Tier 1 networks worldwide, and they form the backbone that everything else eventually connects to. Being Tier 1 is a statement about business relationships and reach — not about speed, quality, or importance to you personally. ## Why "which AS announces your prefix" matters The routing system, BGP, works on trust and announcement. A network says "these addresses are mine" and its neighbours pass that claim along. That single fact has real consequences. * **It tells you who your traffic actually belongs to.** The AS behind your IP is your real network operator — which may not be the brand on your bill, if your provider resells someone else's service. * **It reveals the kind of connection you have.** A residential broadband AS, a mobile carrier AS, and a datacentre hosting AS look very different to a website. Streaming services and banks routinely treat a hosting AS with suspicion, because ordinary customers do not browse from a server rack. This is one of the main ways VPN traffic is detected. * **It affects your apparent location.** Geolocation databases lean heavily on which AS holds a prefix and where that AS operates. See [GeoIP: How IP Geolocation Works](/knowledge-base/concepts/geoip-explained.md). * **It explains routing problems.** If a site is slow or unreachable for you but fine for others, the cause is often a congested or badly chosen link between two specific autonomous systems, not a fault at either end. * **Announcements change, and sometimes wrongly.** A prefix can move between autonomous systems legitimately when addresses are sold or leased. It can also be announced by the wrong network — accidentally through a misconfiguration, or deliberately. This is called a route leak or a BGP hijack, and it is why the history of who announced a prefix is worth looking at. ## Reading an AS in practice When you see an ASN attached to an address, three questions are worth asking. **Who operates it?** The organisation name tells you whether you are looking at a consumer provider, a mobile carrier, a university, a content company or a hosting provider. This one fact often explains everything else on the page. **What kind of addresses does it hand out?** Residential broadband, mobile, business and datacentre ranges behave very differently and are treated very differently by websites. **Where does it operate?** An AS registered in one country may serve customers in several. A mismatch between the AS's registered country and the location reported for the address is worth noticing — it is normal for global operators, and a hint of something else for small ones. | You see | Likely meaning | | -------------------------------------------------------- | --------------------------------------------------------------- | | A well-known consumer ISP | An ordinary home or office connection | | A mobile carrier | Phone or mobile broadband, often shared and imprecisely located | | A hosting or cloud provider | A server, a VPN exit, or a proxy — not a typical end user | | A university or government body | An institutional network | | A name you have never heard, in a country you are not in | Worth a closer look, especially if you expected your own ISP | ## Common misconceptions **"The ASN is part of my IP address."** It is not. The number is stored in routing and registry data, and looked up separately. Two addresses that look similar can belong to entirely different autonomous systems. **"One organisation has one ASN."** Large companies often hold many, one per region or per business unit. Some also acquire the ASNs of companies they buy. **"A bigger AS number means a newer or better network."** The number is just an identifier assigned in sequence. Low numbers usually mean the network registered early, and nothing more. **"My ASN pinpoints my city."** It identifies the network, not your location. A national provider serves an entire country under one or a few ASNs. {% hint style="info" %} **See it yourself on MyIP** The ASN section on your IP card names the autonomous system behind your address, and [ASN Info & Upstream Topology](/knowledge-base/lookup-privacy/asn-info.md) goes further: it shows the historical announcements for your prefix — who has claimed these addresses over time — and draws the upstream paths from your AS toward the Tier 1 backbone networks, so you can see the chain of networks your traffic depends on. {% endhint %} ## Related concepts * [IPv4 vs IPv6](/knowledge-base/concepts/ipv4-vs-ipv6.md) — the addresses that autonomous systems announce * [GeoIP: How IP Geolocation Works](/knowledge-base/concepts/geoip-explained.md) — how routing data feeds location guesses * [Latency, Jitter & Packet Loss](/knowledge-base/concepts/latency-jitter-packet-loss.md) — measuring what the path between networks costs you * [Whois Search](/knowledge-base/lookup-privacy/whois-search.md) — registration records for addresses and domains --- # Agent Instructions This documentation is published with GitBook. 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