An autonomous system number (ASN) is a unique number assigned to a network so that other networks can identify it when they exchange routing information. An autonomous system is a network, or group of networks, run under one routing policy, such as an internet provider, a cloud provider or a large enterprise; the ASN is its label. Networks use their ASNs with Border Gateway Protocol (BGP) to tell each other which IP addresses they can reach, which is how traffic finds its way across the internet.
At a glance
- An autonomous system is a network under one routing policy; the ASN is the number that identifies it.
- ASNs are used by BGP, the protocol networks use to exchange routes.
- Public ASNs are assigned by regional internet registries; private ASNs are reserved for internal use.
- Many businesses never need one; it matters when you run BGP with multiple providers or peer directly with other networks.
- An ASN identifies a network; IP addresses identify the devices and ranges inside it.
What problem it solves
The internet is made of tens of thousands of separately run networks. For traffic to cross between them, each network has to tell its neighbors which address ranges it can deliver to, and those neighbors pass that information on. Routers need a way to tell those networks apart, to know which one originated a route, which networks a route has passed through, and to spot and discard loops. The ASN is that identifier.
For an enterprise, an ASN solves a practical problem: staying reachable at the same public addresses through more than one provider. With its own ASN and address space, a business can announce its addresses to two or more providers at once. If one provider fails, the internet learns to reach those addresses through the other, without the business changing any addresses.
How it works
Assignment. Public ASNs are allocated by the five regional internet registries (RIRs), such as ARIN, RIPE NCC and APNIC, either directly or through a sponsoring member. Each registry has its own policy for who qualifies and what it costs. ASNs were originally 16-bit numbers; 32-bit ASNs were introduced to expand the supply and are now widely used.
Private ranges. Certain ASN ranges are reserved for private use. They are common between a customer and a single provider, inside large private networks, and in some SD-WAN and cloud connection setups. Providers typically remove private ASNs before passing routes to the public internet.
Use in BGP. When a network announces a route, it attaches its ASN. Each network the route passes through adds its own ASN to a list called the AS path. Routers use the AS path to avoid loops and as one input when choosing between routes.
Using it with providers. An enterprise with its own ASN and IP space sets up a BGP session with each IP transit or internet provider. The providers accept its announcements, often after checking registry records or route authorizations, and carry them to the rest of the internet. Networks can also connect directly to each other through peering, privately or at an internet exchange point (IXP).
For help with transit and multi-provider routing, see our IP Transit solution page.
When it matters for buyers
- Multi-provider internet with your own addresses. If you need inbound connections to keep working when one ISP fails, you will usually need an ASN and BGP.
- Running services on your own IP space. Hosting customer-facing services, VPN concentrators or mail servers on addresses you control.
- Data center and colocation deployments. Peering, transit and cloud on-ramps are often set up per ASN.
- Changing providers. With your own ASN and addresses, you can move between transit providers without renumbering.
- Due diligence. Registry (RIR/WHOIS) records show who holds an ASN and its administrative contacts, not its current connectivity. Connectivity is inferred from observed BGP routing data and from self-reported sources such as PeeringDB, and neither shows every connection, but both help when comparing internet providers, including claims about being a Tier 1 network.
Questions to ask vendors
- Do you support BGP with customers that bring their own ASN and IP space, and on which products?
- Will you accept our routes based on registry records and route authorizations, and what documentation do you need?
- Can you provide a private ASN for a single-homed BGP setup if we do not have a public one?
- Which routes will you send us: a full table, a default route only, or both?
- What communities or options do you offer for controlling how our routes are advertised?
- How quickly do you process route changes and new announcements?
How it differs from IP addresses
IP addresses identify individual devices and address ranges; an ASN identifies the network that is responsible for those ranges. A network announces its IP ranges together with its ASN, so the two work together, but they are assigned and used separately. A business can hold its own IP addresses without an ASN, using a provider to announce them, and an ASN is not useful for internet routing without address ranges to announce.