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Anycast

published · September 12, 2026 · updated · October 2, 2026

Anycast announces one IP address from many places at once, so the machine that answers is whichever is nearest on the network, not one fixed location.

Anycast is a way of announcing one IP address from many places at once: the same prefix is routed from dozens or hundreds of sites, and a request lands at whichever site is nearest on the network. The public DNS resolvers (1.1.1.1, 8.8.8.8, 9.9.9.9) and the edge networks of the large CDNs work this way, which is why the address you see is the same in Sydney and in Stockholm while the machine answering is different.

What Spaw answers

is_anycast is true when the address sits in a published anycast range: the bundled resolver ranges shipped with the service, plus the edge ranges Cloudflare and Fastly publish, which carry the anycast signal alongside is_datacenter. It carries no risk weight. Its job is to qualify the location fields: a geolocation database has to put an anycast prefix somewhere, and wherever it chose is at best the nearest instance, not a place a person is. So location_confidence answers low for an anycast address, even where the operator's own geofeed names a city.

What it does not mean

An anycast address is infrastructure, not a visitor. Traffic that arrives from one is unusual — a resolver does not browse — so the flag is rarely the reason a request is interesting. Read it as a warning label on country, city and the coordinates rather than as a signal about whoever sent the request.

Related

markdown version: /glossary/anycast.md

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More glossary

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