An Autonomous System Number is a globally unique identifier for a network that controls its own routing policy. Your ASN tells any website which organisation operates the network your traffic came from, which is a stronger signal about you than your IP address alone.
The definition, without the jargon
The internet is not one network. It is roughly a hundred thousand independently operated networks that agree to exchange traffic. Each of those networks, when it makes its own routing decisions rather than inheriting them from someone else, is called an autonomous system and is assigned an Autonomous System Number.
ASNs are handed out by the five Regional Internet Registries: ARIN for North America, RIPE NCC for Europe and the Middle East, APNIC for Asia-Pacific, LACNIC for Latin America, and AFRINIC for Africa. The numbering was originally 16-bit, giving 65,536 values, and was extended to 32-bit by RFC 6793, which is why modern allocations look like AS398324 rather than AS701.
Autonomous systems announce the IP prefixes they are responsible for using BGP, the Border Gateway Protocol defined in RFC 4271. When your packet crosses the internet, the path it takes is a sequence of ASNs. The last one, the network that originates the announcement covering your address, is the one attributed to you.
An IP address does not contain its ASN. The mapping comes from the global routing table, which changes continuously. A prefix can be re-announced by a different AS within minutes, which is why ASN lookups from different sources occasionally disagree.
How to find your ASN
The most direct method uses Team Cymru's IP-to-ASN DNS service, which answers queries about the current origin AS for any address. The IP is reversed, octet by octet, and appended to origin.asn.cymru.com. A second lookup against asn.cymru.com turns the number into an organisation name.
The example below resolves 1.2.3.4. Substitute your own address, reversed. Alternatively, the WHOIS interface accepts the address directly and returns the same data in one call.
Two caveats are worth knowing. Anycast prefixes are announced from many locations at once, so the ASN is correct while any single location attributed to it is not. And a prefix can legitimately be originated by more than one ASN, which is why some lookups return multiple values rather than one.
- The TXT answer is formatted as: AS number, prefix, country, registry, allocation date.
mtr --aslookupannotates each hop with its ASN, which shows your ISP, its transit providers, and the destination network.- On Windows,
tracerthas no ASN option; usemtrunder WSL or a web-based looking glass instead. - WHOIS at your RIR (
whois -h whois.arin.net 8.8.8.8) returns the registered organisation, which can differ from the announcing AS.
# reverse the octets: 1.2.3.4 becomes 4.3.2.1
dig +short 4.3.2.1.origin.asn.cymru.com TXT
# then resolve the AS number to a name
dig +short AS13335.asn.cymru.com TXT
# single-call alternative
whois -h whois.cymru.com " -v 8.8.8.8"
# see the AS path your packets traverse
mtr --aslookup -rwc 10 1.1.1.1
traceroute -a example.comWhat your ASN reveals
Your ASN identifies the operator of the network, which in turn implies the type of connection. A residential broadband ASN signals a home user on a consumer plan. A mobile carrier ASN signals a phone or a tethered device, often with CGNAT and a wide geographic footprint per address. A cloud or hosting ASN signals a server, a VPN exit, or automation.
That inference is much more stable than geolocation. Individual IP addresses move between customers, get reassigned, and get mislocated. The ASN of an address rarely changes, and when it does, the change is visible in the global routing table within minutes. For a site deciding how much to trust a connection, the ASN is the cheaper and more reliable signal.
It also reveals institutional affiliation in some cases. Universities, large enterprises, and government agencies hold their own ASNs, so a connection from one is attributable to that organisation without any further identification. This is why some employers advise against personal browsing on the corporate network beyond the obvious monitoring concerns.
Reading real ASNs
A short reference makes the categories concrete. The numbers below are stable, widely observed allocations you will encounter constantly when inspecting network data.
Note the pattern: low ASN numbers generally indicate early allocations, which correlate with incumbent carriers and long-established networks. High 32-bit numbers indicate recent allocations, common among hosting providers and newer regional ISPs.
- AS7922 Comcast Cable Communications: US residential and business broadband, one of the largest eyeball networks in the world.
- AS701 Verizon Business (formerly UUNET): a legacy transit and enterprise network; AS22394 covers Verizon Wireless.
- AS20115 Charter Communications and AS22773 Cox Communications: two more large US residential operators.
- AS13335 Cloudflare: anycast edge network; the origin AS for 1.1.1.1 and for a large share of the web's front doors.
- AS15169 Google: serves 8.8.8.8 and Google's own services; AS396982 covers Google Cloud customer traffic.
- AS16509 Amazon: the primary AWS AS, alongside AS14618; a very common origin for bots and VPN exits.
- AS20473 Constant (formerly Choopa, operating Vultr) and AS14061 DigitalOcean: budget cloud hosting, heavily weighted toward automation.
- AS3356 Lumen (formerly Level 3): a tier-1 transit network you will see mid-path rather than as an origin.
When a service tells you it detected a VPN, look up the ASN of the address it saw. If it belongs to a hosting provider rather than a consumer ISP, you have your answer and no amount of clearing cookies will change it.
ASN reputation and why it decides what you can access
Anti-fraud and anti-abuse systems maintain scores at the ASN level, not just the IP level. This is a deliberate design choice: blocking a single abusive address in a cloud provider's range is pointless when the operator can obtain a new one in seconds, so the entire range inherits the reputation of its worst users.
The consequence is collective. A legitimate user renting a server in a range known for scraping will face the same CAPTCHAs, the same streaming blocks, and the same checkout declines as the abusers, because the classification happened before the individual address was considered. Nothing about that user's behaviour caused it.
This also explains an asymmetry that surprises people. Traffic from AS7922 gets a light touch because Comcast's residential range is dominated by ordinary consumers and abuse there is rare relative to volume. Traffic from AS16509 gets heavy scrutiny because the same range hosts both legitimate business integrations and industrial-scale automation, and the site cannot tell them apart at connection time.
If you operate a service and find your outbound traffic being blocked, the practical remedies are to obtain address space with a clean history, to publish accurate WHOIS and rDNS so your identity is verifiable, and to register with the specific platforms that matter to you. Rotating addresses within the same ASN rarely helps, because the score follows the AS.
Frequently asked
Is my ASN the same as my IP address?
No. Your IP address identifies your connection; the ASN identifies the network operator that announces the routing prefix containing that address. Many millions of addresses share one ASN. Your address may change daily while your ASN stays the same for as long as you keep the same provider.
Can I change my ASN?
Not as an ordinary user. Your ASN is determined by whose network you connect through. Changing it means changing ISP, using a VPN or proxy (which substitutes that provider's ASN), or switching to mobile data. Organisations can obtain their own ASN from a Regional Internet Registry, which requires multi-homed connectivity and a justification.
Why do two lookup tools report different ASNs for the same IP?
The IP-to-ASN mapping comes from live BGP data, and different tools sample it from different vantage points at different times. Prefixes can also be announced by multiple ASNs simultaneously, legitimately in anycast deployments or as the result of a routing leak. Team Cymru's data is a good tiebreaker.
Does a hosting ASN mean I will be blocked everywhere?
Not everywhere, but expect more friction. Streaming platforms, ticketing sites, retail checkout flows and some banks apply stricter rules to hosting ranges. General browsing and most APIs are unaffected. The severity depends on the specific provider's abuse history, not simply on being a datacentre.
How many ASNs exist?
Roughly 120,000 have been allocated by the Regional Internet Registries, of which approximately 80,000 are actively announcing routes in the global table at any given time. The 16-bit space of 65,536 numbers was exhausted, which is why 32-bit ASNs defined in RFC 6793 are now standard for new allocations.