SKNK Technical Guide
What Is an ASN? A Complete Beginner's Guide to Autonomous System Numbers
Learn what an Autonomous System Number (ASN) is, why the Internet depends on it, how it works with BGP, and when your organization actually needs one.
Technical content
Written and maintained by Shikanoko NetworksIntroduction
Every time you open a website, send an API request, or stream a video, your traffic crosses multiple independent networks before reaching its destination.
Those networks don't identify each other by domain names — or even by IP addresses.
Instead, they identify each other using Autonomous System Numbers (ASNs).
Although most Internet users never notice them, ASNs are one of the fundamental building blocks that make global Internet routing possible.
The answer lies in how the Internet is actually built: not as a single unified network, but as a collection of thousands of independently operated networks — each making its own routing decisions, each controlled by a different organization.
An Autonomous System Number (ASN) is a globally unique identifier assigned to an Autonomous System — a network or group of IP networks managed under a single routing policy. It identifies an entire network when that network communicates with the rest of the Internet.
Without ASNs, the modern Internet simply could not exchange routing information efficiently between millions of networks around the world.
Whether you're a network engineer, cloud architect, SaaS founder, or simply curious about how Internet routing works, understanding ASNs is one of the best foundations for learning modern networking.
The Internet works because independent networks can identify one another and exchange routing information.
What Is an ASN?
An Autonomous System Number (ASN) is a unique numerical identifier assigned to an Autonomous System (AS).
An Autonomous System is a collection of one or more IP networks operated by a single organization that presents a consistent routing policy to other networks on the Internet.
Here's an easier way to visualize the distinction:
- An IP address identifies where data should ultimately go — the destination.
- An ASN identifies which network is responsible for advertising and routing that destination.
Think of it this way:
- Your home address tells people where your house is.
- Your city's transportation department decides how roads connect to other cities.
Similarly:
- An IP address identifies a device or network location.
- An ASN identifies the network operator responsible for routing traffic to and from that location.
This distinction matters because the Internet is not built from individual computers communicating directly with each other. It is built from thousands of independent networks exchanging routing information. Those networks need identities of their own — and that identity is the ASN.
What Is an Autonomous System?
To understand ASNs, you first need to understand what an Autonomous System actually is.
An Autonomous System (AS) is a network — or a collection of networks — that is operated by one organization and follows a single routing policy when communicating with external networks.
That organization could be:
- An Internet Service Provider (ISP)
- A cloud provider
- A content delivery network (CDN)
- A university
- A government network
- A large enterprise
- A hosting company
For example, global organizations operate enormous networks consisting of thousands of routers and data centers spread across multiple countries. Although these internal networks may be extremely complex, other networks on the Internet don't need to understand every router inside them.
Instead, they simply see one Autonomous System announcing its reachable IP prefixes.
This abstraction is one of the reasons the Internet can scale. If every router had to know the internal topology of every network on Earth, global routing would become unmanageable. Instead, each Autonomous System hides its internal design and exchanges only the information that other networks actually need.
Why Does the Internet Need ASNs?
The Internet is often described as a "network of networks." That description is more accurate than many people realize.
Every ISP, cloud provider, enterprise backbone, and Internet exchange participant operates its own network. These networks constantly exchange routing information so that traffic can travel across organizational boundaries.
A single request may cross several organizations before it reaches the service you wanted.
Each step belongs to a different organization. Each organization controls its own routers. Each organization makes its own routing decisions.
The Internet therefore needs a way for these independent networks to identify one another. That's exactly what ASNs provide.
Instead of exchanging information about millions — or even billions — of individual devices, routers exchange routing information between Autonomous Systems. This dramatically reduces complexity while allowing each network to manage its own infrastructure independently.
How ASN Works with BGP
ASNs become useful because of the Border Gateway Protocol (BGP), the routing protocol responsible for exchanging routes between Autonomous Systems.
When a network wants the rest of the Internet to know it can reach a particular IP prefix, it advertises that information through BGP.
A simplified route advertisement might look like this:
203.0.113.0/24
Origin ASN: AS64500
This tells neighboring networks: "Traffic destined for this IP prefix can be reached through AS64500."
As that route spreads across the Internet, routers build routing tables containing information such as:
- IP prefix — the destination network block
- Origin ASN — the Autonomous System that originated the route
- AS Path — the sequence of Autonomous Systems the route passed through
- Next Hop — the next router in the path toward the destination
- Other routing attributes — such as local preference, MED, and community values
Notice something important: routers are not making decisions based on individual servers. They are making decisions based on relationships between Autonomous Systems.
This is why ASNs are fundamental to Internet routing. Without them, BGP would have no standardized way to identify which network originates or forwards a route.
BGP lets networks announce reachability and build paths across Autonomous Systems.
BGP path selection is covered in detail in our upcoming networking guides.
ASN vs IP Address: What's the Difference?
One of the most common misconceptions is that an ASN and an IP address serve the same purpose. They don't. Although they are closely related, they solve entirely different problems.
| Feature | ASN | IP Address |
|---|---|---|
| Identifies | A network (Autonomous System) | A device or network interface |
| Used by | BGP | IP (Internet Protocol) |
| Scope | Network-wide routing policy | Individual host or interface |
| Assigned by | Regional Internet Registry (RIR) | Allocated from IP blocks by RIR/LIR |
| Changes frequently | No — typically stable for years | Sometimes — reassignments, renumbering |
| Exchanged in | BGP UPDATE messages | IP packet headers |
| Purpose | Tells other networks who is routing | Tells other devices where to deliver |
A simple analogy:
Imagine sending a package to another country.
The destination address on the package is like an IP address — it tells the delivery service where the package ultimately needs to arrive.
The shipping companies, airports, customs agencies, and transportation networks responsible for moving the package resemble Autonomous Systems. Each organization decides how to forward the package until it reaches its final destination.
Similarly, routers use IP addresses to identify destinations, but they rely on ASNs to exchange routing information between independent networks.
An IP address points to where traffic should go; an ASN identifies who routes it.
How ASNs Relate to IP Prefixes
When you first encounter networking terminology, it's easy to mix up ASNs, IP prefixes, CIDR notation, IPv4, and IPv6. They are related but distinct concepts.
| Concept | What It Represents | Example |
|---|---|---|
| ASN | The network operator's identity | AS64500 |
| IP Prefix | A block of IP addresses announced by an ASN | 203.0.113.0/24 |
| CIDR Notation | The format used to express a prefix and its size | /24 = 256 addresses |
| IPv4 | The 32-bit addressing scheme most of the Internet still uses | 192.0.2.1 |
| IPv6 | The 128-bit addressing scheme designed to replace IPv4 | 2001:db8::1 |
The relationship works like this:
- An organization obtains an ASN from an RIR.
- The organization obtains IP prefixes (blocks of IPv4 or IPv6 addresses).
- Using BGP, the organization announces those prefixes with its ASN attached.
- Other networks learn: "Prefix X can be reached via ASN Y."
In other words:
ASN = who is announcing. IP Prefix = what is being announced.
A prefix becomes reachable when an Autonomous System announces it to the rest of the Internet.
This is why you'll often hear the phrase "BGP prefix announcement." A prefix without an ASN is just a block of addresses sitting in a database. An ASN without prefixes has nothing to route. They only become meaningful together.
IP prefixes and CIDR notation will be covered in detail in our upcoming networking guides.
Public ASN vs Private ASN
Not every Autonomous System participates directly in the global Internet. For this reason, ASNs are divided into two categories.
Public ASN
A public ASN is globally unique and can participate in Internet routing through BGP.
Organizations that announce their own IP prefixes to external networks typically require a public ASN. Examples include:
- Internet Service Providers
- Cloud providers
- Content delivery networks
- Internet exchange participants
- Large enterprises operating multiple upstream connections
Public ASNs are allocated by the appropriate Regional Internet Registry (RIR) according to its current policies.
Private ASN
Private ASNs are intended for internal use.
They are commonly used inside organizations that run BGP internally or connect to another provider without requiring a globally unique ASN. Unlike public ASNs, private ASNs are not intended to appear in global Internet routing tables. Before routes are advertised to the public Internet, private ASNs are typically removed or replaced by the upstream provider.
For many enterprise environments, a private ASN is entirely sufficient.
Public ASNs are visible in global routing; private ASNs belong inside controlled environments.
16-bit vs 32-bit ASNs
When BGP was first standardized, ASNs were 16 bits long. This allowed approximately 65,000 possible values — more than enough for the Internet at the time.
As the number of networks continued to grow, the available ASN space became increasingly limited. To support long-term growth, the Internet transitioned to 32-bit ASNs.
Today, both formats coexist, and modern routers fully support 32-bit ASNs.
For most administrators, the distinction has little impact on day-to-day operations. Whether an ASN contains four digits or ten digits, it serves exactly the same purpose: uniquely identifying an Autonomous System.
Who Assigns ASNs?
ASNs are coordinated globally to ensure that each public ASN remains unique.
At the top of this hierarchy is the Internet Assigned Numbers Authority (IANA), which manages global number resources.
IANA delegates ASN allocation to five Regional Internet Registries (RIRs), each responsible for a specific geographic region:
| RIR | Region |
|---|---|
| AFRINIC | Africa |
| APNIC | Asia-Pacific |
| ARIN | North America |
| LACNIC | Latin America and the Caribbean |
| RIPE NCC | Europe, the Middle East, and parts of Central Asia |
Each RIR maintains its own policies, documentation, and application process. Although the overall purpose is the same, eligibility requirements and administrative procedures may differ slightly between regions.
If you're considering applying for a public ASN, it's always best to consult the policy documents published by the RIR responsible for your region.
If you're in the RIPE NCC service region, read our detailed comparison: RIPE NCC vs LIR Sponsorship: What Nobody Tells You.
ASNs in the Real World: A Packet's Journey
Instead of listing which companies have ASNs (spoiler: all of them), let's look at what actually happens when you use the Internet.
When you visit youtube.com
Your browser doesn't know or care about ASNs. But the routers between you and YouTube absolutely do:
Your browser sees a website. The routers underneath see a path across multiple networks.
At every step between "Local ISP" and "Google's network edge," BGP routers are making decisions based on AS Paths — not IP addresses alone. Your ISP's router doesn't need to know where every YouTube server lives. It only needs to know: "To reach Google's prefixes, send traffic toward Google's ASN via this transit provider."
When you deploy on a cloud platform
Even if you never touch BGP yourself, your cloud provider is using ASNs constantly:
- AWS, Google Cloud, and Azure each operate multiple Autonomous Systems.
- When you assign a public IP to a VM, that IP belongs to a prefix announced by your provider's ASN.
- Traffic between cloud regions travels across BGP-managed paths between the provider's Autonomous Systems.
You benefit from ASNs every day — you just don't see them.
Can a Company Have More Than One ASN?
Yes — and it's more common than you might think.
Large organizations frequently operate multiple Autonomous Systems. Common reasons include:
- Geographic separation — different regions may be managed under distinct ASNs for operational or regulatory reasons.
- Business units — separate divisions within a large corporation may operate their own networks independently.
- Mergers and acquisitions — when one company acquires another, the acquired company's existing ASNs may continue to operate alongside the parent organization's ASNs.
- Different routing policies — an organization might use separate ASNs for different services, such as separating its transit business from its hosting business.
- Traffic engineering — multiple ASNs can provide more granular control over how traffic enters and exits the network.
However, operating multiple ASNs adds administrative overhead. For most small and mid-sized organizations, a single ASN is more than sufficient.
How to Find an ASN
One of the most common practical questions about ASNs is simply: "How do I look one up?"
Whether you're troubleshooting a routing issue, researching a network, or just curious which ASN your traffic is passing through, there are several free tools available.
WHOIS Lookup
Every public ASN is registered in a WHOIS database. You can query it directly:
whois AS15169
This returns the ASN's owner, registration date, contact information, and associated IP prefixes. Each RIR maintains its own WHOIS server, but most whois clients will automatically query the correct one.
BGP Looking Glass
Looking Glasses are public BGP route servers operated by major networks. They let you see how a specific ASN or IP prefix appears from that network's perspective.
Popular Looking Glasses include:
- Hurricane Electric BGP Toolkit (bgp.he.net) — search by ASN, IP, or prefix; view peers, prefixes, and upstream providers.
- RIPE RIS (stat.ripe.net) — RIPE NCC's routing information service with historical data.
- RouteViews (routeviews.org) — University of Oregon's BGP route collection project, useful for research and debugging.
RIPE Database
If you're in the RIPE NCC service region, the RIPE Database is the authoritative source for ASN and IP resource registrations. It's also where you'll manage your own ASN object after obtaining one.
Quick lookup commands
For a fast terminal check:
# WHOIS an ASN
whois -h whois.ripe.net AS64500
# Check which ASN announces a given IP
whois 8.8.8.8 | grep -i origin
# Trace the AS path to a destination (requires BGP-capable router or looking glass)
Tip: You can also use a BGP Looking Glass (such as Hurricane Electric's BGP Toolkit) or run a traceroute and cross-reference the IP hops with WHOIS to see which ASN each hop belongs to.
Do You Need Your Own ASN?
For many organizations, the answer is simply no.
Owning an ASN does not automatically improve network performance, increase reliability, or make a website faster. If you host applications on a single cloud provider, use standard Internet connectivity, or rely entirely on managed networking services, you may never need your own ASN.
However, there are situations where operating an Autonomous System becomes genuinely beneficial:
- Connecting to multiple upstream Internet providers (multi-homing) for redundancy
- Advertising your own IP address space independently
- Implementing custom BGP routing policies for traffic engineering
- Building geographically distributed Anycast services
- Operating a large enterprise or service provider network
The decision to get an ASN should come from routing requirements, not prestige.
In these cases, an ASN allows your network to exchange routing information directly with other Autonomous Systems rather than relying entirely on a single upstream provider.
The decision should be based on operational requirements — not curiosity, perceived prestige, or the assumption that "owning an ASN" is inherently valuable.
Wondering about costs? We break down the actual numbers in €89 vs €2,850: The Real Cost of Running Your Own ASN.
Common Misconceptions
Several myths continue to appear in networking discussions. Let's address them directly.
"An ASN makes my network faster."
Not directly. Network performance depends on factors such as physical distance, routing policy, peering relationships, congestion, and link capacity. An ASN is an identifier — it does not reduce latency by itself.
"Every company needs an ASN."
Most organizations do not. Many successful businesses operate entirely on cloud platforms without ever managing BGP or obtaining a public ASN.
"You need an ASN to host a website."
You don't. Millions of websites run perfectly well using IP addresses and networking services provided by hosting companies or cloud providers. The web server and the network layer are separate concerns.
"An ASN and an IP address are interchangeable."
They are fundamentally different. An IP address identifies where traffic should go. An ASN identifies which network is responsible for routing that traffic.
"Getting an ASN is complicated and expensive."
It depends on your route. Through a Sponsoring LIR, obtaining an ASN in Europe can be straightforward and cost a fraction of what direct RIPE NCC membership requires. The complexity comes from what you choose to do with the ASN — not from obtaining the number itself.
Frequently Asked Questions
What does ASN stand for?
ASN stands for Autonomous System Number — a globally unique identifier assigned to an Autonomous System.
Is an ASN required for BGP?
In external BGP (eBGP), Autonomous System Numbers are required to identify the neighboring networks exchanging routing information between independently managed Autonomous Systems. Internal BGP (iBGP) operates within the same Autonomous System and uses the organization's existing ASN.
Can an individual obtain a public ASN?
This depends on the policies of the relevant Regional Internet Registry. Eligibility requirements vary by region and may change over time. Consult your local RIR's current policy documentation for the most accurate guidance.
Does owning an ASN improve Internet speed?
No. Performance improvements come from network design, routing policies, peering arrangements, and infrastructure — not from the ASN itself.
Do I need my own ASN to use cloud services?
In most cases, no. Cloud providers already operate their own Autonomous Systems and manage Internet routing on behalf of their customers. Unless you need to advertise your own IP space or connect to multiple providers independently, your cloud provider handles ASN-level concerns for you.
What's the difference between an ASN and an IP prefix?
An ASN identifies who is routing. An IP prefix identifies what is being routed. They work together in BGP: an ASN originates a route announcement for one or more IP prefixes.
How can I check which ASN my traffic is going through?
Use a BGP Looking Glass (such as Hurricane Electric's BGP Toolkit) or run a traceroute and cross-reference the IP hops with WHOIS to see which ASN each hop belongs to. For a quick check, whois <IP> and look for the origin field.
Conclusion
Most Internet users will never configure BGP or apply for an ASN.
Yet every website they visit, every API request they send, and every cloud service they use depends on thousands of Autonomous Systems exchanging routes behind the scenes.
An ASN identifies entire networks and allows them to exchange routing information through BGP. This abstraction — each network hiding its internal complexity while announcing only what other networks need to know — is what enables the Internet to scale beyond any single organization's control.
Understanding ASNs isn't just about learning another networking acronym. It's about understanding one of the core design decisions that allows the modern Internet to function across millions of independently operated networks.
For most organizations, understanding what an ASN is matters more than owning one. Whether you're deploying cloud infrastructure, learning BGP, or designing a large-scale network, knowing how Autonomous Systems fit into Internet routing provides the foundation for everything that follows.
Key Takeaways
- An ASN identifies an entire network, not an individual device.
- Autonomous Systems exchange routing information through BGP.
- Most organizations do not need their own ASN.
- Public and private ASNs serve different purposes.
- An ASN does not automatically improve network performance.
Continue Reading
- €89 vs €2,850: The Real Cost of Running Your Own ASN — a detailed cost breakdown of ASN ownership in Europe.
- RIPE NCC vs LIR Sponsorship: What Nobody Tells You — which path actually makes sense for your organization.