See the ASN behind any address on /ip-lookup, and the one behind your own on the full report.
What an autonomous system is
The internet is not one network. It is roughly 80,000 independently operated networks that agree to exchange traffic. An autonomous system is one of those: a set of IP prefixes under a single technical administration with one routing policy. RFC 1930 sets out when an organisation actually needs its own number.
Autonomous systems talk to each other with the Border Gateway Protocol, RFC 4271. A BGP announcement says, in effect, “reach 203.0.113.0/24 through me”, and carries the AS path the announcement travelled. Your ISP’s routers pick a path for every destination out of the announcements they receive.
That is why the ASN is trustworthy in a lookup. Every router carrying your traffic needs it to be correct. A city name in a geolocation database has no such enforcement.
16-bit and 32-bit numbers
ASNs started as 16-bit values, giving 65,536 of them. That ran short, so RFC 6793 extended the space to 32 bits, giving about 4.3 billion.
| Range | Bits | Meaning |
|---|---|---|
| 0 | 16 | Reserved |
| 1 to 64,495 | 16 | Public, assigned by the RIRs |
| 64,496 to 64,511 | 16 | Documentation, RFC 5398 |
| 64,512 to 65,534 | 16 | Private use, RFC 6996 |
| 65,535 | 16 | Reserved, RFC 7300 |
| 65,536 to 65,551 | 32 | Documentation, RFC 5398 |
| 65,552 to 4,199,999,999 | 32 | Public, assigned by the RIRs |
| 4,200,000,000 to 4,294,967,294 | 32 | Private use, RFC 6996 |
| 4,294,967,295 | 32 | Reserved, RFC 7300 |
RFC 6996 reserves the two private ranges. They work like RFC 1918 addresses: use them inside your own routing domain, strip them before announcing anything to the internet, and expect collisions with whoever else picked the same number. The 16-bit private block holds only 1,023 numbers, which is why the much larger 32-bit block was added.
One number you will see in old captures is 23456, called AS_TRANS. A 16-bit-only router substitutes it when it cannot represent a 32-bit AS number in a path. It should never appear as an origin.
Who hands them out
IANA allocates blocks of ASNs to the five regional internet registries, and each RIR assigns them to organisations in its service region.
| RIR | Region |
|---|---|
| ARIN | United States, Canada, parts of the Caribbean |
| RIPE NCC | Europe, Middle East, Central Asia |
| APNIC | Asia Pacific |
| LACNIC | Latin America and the Caribbean |
| AFRINIC | Africa |
The registry that assigned a number also holds its WHOIS or RDAP record, which names the operator and lists technical and abuse contacts. Query it directly on /whois-lookup. IANA publishes the top-level allocation in its AS Numbers registry.
How to read an ASN in a lookup result
A result gives you a number, a name, and often a network type. What each part is worth:
- The number is definitive. It is what the routing table announced when the lookup ran.
- The name comes from the registry record and lags reality. Mergers and rebrands land in the record months late, or never.
- The type, such as ISP, hosting, business, or education, is a vendor classification rather than a registry field. It is reliable for large hosting networks and mobile carriers, softer for small ones.
Practical uses. A hosting ASN behind an address that claims to be a residential visitor usually means a VPN, a proxy, or a scraper. A mismatch between the ASN on your IPv4 address and the one on your IPv6 address means one protocol is tunnelled and the other is not, which is the check on /ipv6-test. And an abuse contact on the ASN record is the correct place to report traffic from a network, rather than the hosting customer.
Notation, and the numbers you will recognise
Write them as AS followed by the number with no space: AS15169, AS13335. The dotted form for 32-bit numbers, 1.10 for 65546, appears in older Cisco output and in some RIR documents. It is the same value in a different notation.
A few of the largest are worth recognising because they appear constantly in lookups: AS15169 is Google, AS13335 is Cloudflare, AS16509 is Amazon, AS8075 is Microsoft, and AS32934 is Meta. An address in any of those is a data centre, not a household.
You can check the announcements behind any number yourself with public route collectors such as RIPE RIS and RouteViews, which publish the raw BGP tables that every lookup service reads.
Questions people ask
- How do I find the ASN for my IP address?
- Open the full report on this site, or paste the address into /ip-lookup. The AS number and the AS name appear together, taken from the global routing table rather than from a geolocation guess.
- Do I need my own ASN?
- Only if you run BGP. You need one to connect to more than one upstream provider, to peer at an exchange, or to announce address space you hold. A single-homed network uses its provider's addresses and needs nothing.
- What is the difference between an ASN and an IP address?
- An IP address identifies one interface. An ASN identifies the organisation that decides how traffic reaches a whole set of address blocks. One ASN typically announces hundreds or thousands of prefixes.
- Why does my ASN name not match my ISP's brand?
- Networks are bought, merged, and rebranded far more often than registry records are updated, and consumer brands frequently run on a parent company's network. The AS name is the registered operator, not the marketing name.
Related
Last reviewed 2026-09-04. Reviewed quarterly, or sooner when a vendor changes something.