ASN Lookup & AS Reputation
Enter an IPv4 or IPv6 address to find the Autonomous System that announces it in BGP — the AS number, the network's name, the announced prefix, and the country/registry — plus its reputation: whether the whole AS is on the Spamhaus ASN-DROP list (a network run by spammers/criminals) or UCEPROTECT Level 3 (a network-wide spam reputation). It's the quick way to see who routes an IP and whether that network is trustworthy.
Reviewed by the SysGlass team · Updated July 21, 2026
What it checks
- The origin AS number (ASN) that announces the IP in BGP
- The network/organization name for that AS
- The announced BGP prefix that contains the IP
- The country and the regional internet registry (ARIN, RIPE, APNIC, ...)
- AS reputation: Spamhaus ASN-DROP (criminal network) and UCEPROTECT Level 3 (network-wide spam)
- Works for IPv4 and IPv6
How to read the result
- OK shows the AS number, network name and prefix — e.g. AS13335 (CLOUDFLARENET), 1.1.1.0/24.
- The network name tells you who operates the IP (an ISP, a cloud provider, a hosting company).
- The prefix is the block being announced; other IPs in that prefix belong to the same network.
- 'Not announced in BGP' means no origin AS was found — the IP may be unrouted, reserved/bogon, or private.
Frequently asked questions
What is an ASN?
An Autonomous System Number identifies a network that announces routes in BGP — typically an ISP, cloud provider, or large organization. Every public IP is announced by some origin AS.
How do you look up the ASN for an IP without an API key?
This tool uses Team Cymru's DNS-based IP-to-ASN service, which answers ASN queries over standard DNS — no account or API key needed.
What's the difference between the ASN and the prefix?
The ASN identifies the network operator; the prefix (e.g. 1.1.1.0/24) is the specific block of addresses that AS is announcing. The prefix contains your IP and many of its neighbors.
Why does it say the IP is not announced?
Some IPs aren't routed on the public internet — private ranges, reserved/bogon space, or addresses not currently advertised in BGP. There's no origin AS for those.
Does it work for IPv6?
Yes — enter an IPv6 address and it returns the announcing ASN, name and prefix just like IPv4.
Can you check an ASN's spam reputation?
Yes. We cross-reference the AS against Spamhaus ASN-DROP (entire networks operated by spammers/cybercriminals) and UCEPROTECT Level 3 (networks with excessive spam). Both are free, authoritative public sources — no vendor-locked score. Each source is best-effort: if one is temporarily unavailable we say so rather than guess.
My IP is clean but its AS is listed — does that matter?
It can. Some receivers penalise every IP inside a network that's on ASN-DROP or UCEPROTECT Level 3, so a bad-neighborhood AS can drag down a clean IP's deliverability. If that's you, consider sending from a better-reputation network.