Looking up…
What an AS number is.
An autonomous system is a network with one routing policy: an internet provider, a cloud, a university, a large company. Networks swap routes with each other by AS number over BGP. IANA hands blocks of numbers to the five regional registries, which assign them to networks. The first numbers had 16 bits, up to 65,535; four-byte numbers now run past four billion.
Reading the prefixes.
- A prefix is an address block the network tells the rest of the internet it carries, written as an address and a length:
104.16.0.0/13covers 524,288 addresses. - Hivex reads them from RIPEstat, which counts a prefix when at least 10 of RIPE's route collector peers saw it in the last day. A prefix seen by fewer is left out, which hides some purely local announcements.
- A network that holds a number but announces nothing is assigned but not routing traffic: kept for later, or used only inside a private network.
Numbers you will not find.
64512 to 65534 and 4200000000 to 4294967294 are kept for private use inside a company's own network and never appear on the internet. AS23456 is the stand-in older routers see in place of a four-byte number. You can also type an IP address or a domain: the tool finds the network that announces it.
Questions people ask.
What is an ASN?
An autonomous system number identifies a network that routes its own addresses on the internet, such as an internet provider, a cloud or a large company.
How do I find the ASN of an IP address?
Type the IP address: the lookup finds the AS that announces it, then shows that network.
What does it mean that a network announces a prefix?
It tells its neighbours, through BGP, that it delivers traffic for that block of addresses. The prefixes shown are the ones seen announced today.
Who assigns AS numbers?
IANA gives blocks of numbers to the five regional internet registries, which assign them to networks in their regions.
Hivex index
Short names, still free to register.
Starting something new? Hivex keeps a live index of short, brandable .si names nobody has claimed yet, each checked with the registry.
From code, or an AI assistant.
Hivex's free JSON API and MCP server check domains, DNS and registration records from your own code or from AI assistants that speak MCP. No key needed.
More Network tools
- Ping testLatency to a site or IP: five TCP handshakes, timed.
- Open port checkerIs a port open on your connection? Tested from outside.
- What is my IPYour public IP, approximate location, provider and connection.
- IP lookupOwner, network, AS number and hostname of any IP address.
- Speed testDownload, upload and latency, in the browser.
- Domain checkerIs a name free to register? Checked live on any extension.
Sources
- RFC 1930: Guidelines for creating, selecting and registering an autonomous system
- RFC 6793: BGP support for four-octet AS numbers
- RFC 6996: AS numbers reserved for private use
- IANA: autonomous system numbers registry
- RIPEstat: announced prefixes
By Hivex. Updated 10 October 2026. Lookups run live through Cloudflare's public resolver and are kept for at most five minutes.