An Autonomous System Number (ASN) is a globally-unique identifier for a network operator on the public internet. Every ISP, hosting provider, content network, and large-enough organization that runs its own routing announcements has one. Google's network is AS15169, Cloudflare's is AS13335, Comcast's residential broadband is AS7922.

ASNs are how the internet's outer layer of routing — the Border Gateway Protocol, BGP — describes who controls which blocks of IP addresses. When you load a web page, packets hop between autonomous systems, each one announcing to its neighbors which prefixes (ranges of IP addresses) it can deliver traffic for. Mapping an IP back to its ASN tells you which network operator is announcing that address right now.

ASNs are assigned by the same five Regional Internet Registries that hand out IP-address blocks: ARIN (North America), RIPE NCC (Europe, Middle East, parts of Central Asia), APNIC (Asia-Pacific), LACNIC (Latin America and the Caribbean), and AFRINIC (Africa). Each registry publishes its allocations in a standardized whois database, which is where the human-readable holder name on this page comes from.

Every query you run on this page is a request from your browser directly to a public data source. Mungomash never sees the IP you typed — there is no server-side code involved on this page.

Primary source — RIPE Stat. The headline lookup (ASN, prefix, holder, country, registry) is sourced from RIPE Stat, an open-data service run by the RIPE NCC. RIPE Stat aggregates BGP routing data and the five regional registries' whois databases into a single JSON API, free for public use, no key required. A handful of small endpoints (network-info, as-overview, rir, whois) get queried in parallel for one round-trip.

Fallback source — Team Cymru. If RIPE Stat is unreachable or rate-limiting, the page falls back to Team Cymru's IP-to-ASN service, a free DNS-based BGP lookup. The query is one DNS TXT record, retrieved through the same DNS-over-HTTPS pipe used by /tools/dns/. The answer set is smaller (no detailed whois, no organizational record) but the headline ASN, prefix, country, and registry come back fast.

Reverse PTR. The reverse-DNS PTR record — what name, if any, the IP's network operator has published for it — is looked up via Cloudflare's DNS-over-HTTPS endpoint. PTR records live in in-addr.arpa (IPv4) or ip6.arpa (IPv6); not every IP has one, and the answer is purely advisory.

Number resources — IP addresses and ASNs — are administered globally by the IANA, which delegates regional administration to five non-profit registries. The registry your IP belongs to determines which whois database holds its record and which legal regime governs its allocation.

• No city / lat-lon geolocation. IP-to-city mapping needs commercial data (MaxMind and friends), is frequently wrong by hundreds of miles, and is a privacy concern when surfaced authoritatively. Country-level info (from the registry's whois) is the most precise location this page will publish.
• No abuse / threat-reputation lookups. Services that score the badness of an IP (Spamhaus, AbuseIPDB, etc.) have restrictive terms for ad-supported display. Out of scope here.
• No subnet calculator. Computing network/broadcast/host counts for an arbitrary CIDR block is a different mental model. A separate /tools/subnet/ page would be the right home if the need shows up.
• No "all prefixes for this ASN" listing. Some lookup tools dump every prefix an ASN announces. That's useful for network engineers but mostly noise for the "who owns this IP?" question this page targets. bgpview.io and stat.ripe.net both publish this view if you need it.