Anycast

An anycast prefix is announced from several places at once, so the same IP address answers from different physical locations depending on where you ask from. It is how large DNS and CDN operators put a single address everywhere.

Where the data comes from

Atlas does not infer anycast from its own measurements. It imports the IPv4 and IPv6 anycast-prefix feeds published by manycast.net on a weekly cycle, and stamps the result onto its own IP enrichment records so an IP lookup can say "this address is in a known anycast prefix".

That matters when you are judging the data: the detection quality, the update cadence and the false-positive profile are manycast's, not ours. Atlas's contribution is keeping the import current, tracking what changed between imports, and joining it to everything else it knows about a prefix.

Anycast views are staff-gated.

Reading the confidence scores

Each prefix carries two families of numbers from the upstream feed.

AnyBest probability (AB_ICMP, AB_TCP, AB_DNS) is a 0–100 score per protocol — higher means more confident the prefix is anycast. Three separate protocols are scored because a prefix can behave differently depending on what you probe it with: an operator may anycast their DNS service while the same prefix answers ICMP from one site.

Great-circle dispersion (GCD_ICMP, GCD_TCP) measures how geographically spread the observed catchments are. A high AnyBest score with low dispersion suggests several sites close together; high on both is the classic global anycast deployment.

Take the scores together, not individually. One high protocol score with the others near zero is worth a second look before you rely on it.

Partial prefixes

A prefix flagged partial is one where only part of the address space is actually anycast. The record then also names a backing prefix, the covering announcement it sits inside. Treat "this /24 is anycast" as a statement about the prefix as a whole, not a guarantee about every address in it.

Catchments

Each prefix lists the locations where it was observed answering — city, country and coordinates — with counts for distinct locations and distinct countries. This is the most directly useful part for capacity and routing questions: it tells you where a service actually answers from, as observed, rather than where its operator says it has presence.

Catchments are observations from a fixed vantage-point set. Absence of a location is weak evidence: it may mean no site there, or it may mean nobody looked from nearby.

What it cannot tell you

• Which site you reach. Catchment data is aggregate. Your own path is a routing question — use a traceroute measurement or routing history.
• Anything sub-weekly. A site that came up on Tuesday shows up after the next import.
• Newly anycast prefixes. A prefix only appears once the upstream has observed it, so recent deployments lag.

Atlas records the add/remove diff between imports, so a prefix appearing or disappearing is itself visible as a change rather than a silent edit.

Where it shows up

• Anycast prefixes (/network/anycast) — the browsable list, and per-prefix detail with sites and catchment.
• Anycast intel (/network/anycast/intel) — the enriched cross-referenced view.
• IP lookup (/network/ip) — an anycast marker on any address inside a known anycast prefix.

See the Network guide.