How to use ASN Lookup
The ASN Lookup tool maps an IP address — or an autonomous system number you already have — to the network that announces it on the global routing table. Every block of public IP space is originated by an Autonomous System (AS), identified by a number such as AS13335, and operated by an ISP, hosting company, or large enterprise. Looking up the ASN tells you who really runs the address: the network name, the specific BGP prefix the address falls inside, the regional internet registry that assigned it, and the country of allocation. It is an essential first step when investigating abuse, profiling infrastructure, or understanding how traffic is routed across the internet.
- Enter an IPv4 or IPv6 address, or an AS number like AS15169.
- Click Lookup to query the IP-to-ASN routing data.
- Read the originating ASN, network name, and announced prefix.
- Note the assigning registry and country for the allocation.
- Use the AS number to pivot into further routing or WHOIS research.
How IP-to-ASN mapping works
Autonomous Systems advertise the IP prefixes they control using the Border Gateway Protocol (BGP). Public route collectors observe these advertisements and record which ASN originates each prefix. To map an IP to its ASN, this tool queries Team Cymru’s IP-to-ASN service over DNS, which returns the origin ASN, the most specific prefix covering the address, the assigning registry, and the allocation date. Because the data is derived from live BGP, it reflects how the address is actually routed today rather than a static ownership database.
ASN vs WHOIS — what each tells you
An ASN identifies the network that routes an address, while IP WHOIS records the organization an address block is registered to. They often point to the same company, but not always: a customer may use IP space announced from their provider’s ASN, or a large operator may route many registered organizations. Checking both gives a fuller picture — the ASN shows who carries the traffic, and WHOIS shows who is administratively responsible.
| Registry | Region |
|---|---|
| ARIN | North America |
| RIPE NCC | Europe & Middle East |
| APNIC | Asia-Pacific |
| LACNIC | Latin America |
| AFRINIC | Africa |
Glossary
- ASN
- Autonomous System Number — a unique identifier for a network on the internet.
- BGP
- Border Gateway Protocol, used by networks to advertise the IP prefixes they route.
- Prefix
- A block of IP addresses announced together, written in CIDR form like 1.1.1.0/24.
- RIR
- Regional Internet Registry that allocates IP space and ASNs within a region.
- Origin AS
- The autonomous system that originates a prefix into the global routing table.
Related reading
Frequently Asked Questions
Why use ASN Lookup?
- Identify the network operator and ASN behind any public IP address
- See the exact BGP prefix that an address is announced from
- Find the regional registry (ARIN, RIPE, APNIC) and country of allocation
- Pivot from an IP to its AS number for deeper routing investigation
Common use cases
- Determine which hosting provider or ISP owns an IP seen in server logs
- Investigate the network behind a suspicious or abusive address
- Confirm the ASN of a CDN edge node when debugging routing or geolocation
- Map the prefixes and registry of a competitor’s hosting infrastructure
- Verify that an IP belongs to the ASN you expect before allow-listing it
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