Anycast

Anycast is a routing technique where the same IP address is advertised from multiple, geographically separate locations, and ordinary internet routing (BGP) delivers each client's traffic to whichever announcement is topologically nearest — with no client-side awareness that a choice was even made. It's a one-to-nearest-of-many relationship, distinct from unicast (one-to-one) and Multicast (one-to-every-subscriber).

The mechanism is almost boring in its simplicity: routers already pick the shortest path to a destination as their basic job, and anycast just exploits that by making "the destination" ambiguous — several data centers claim to be address X, and each router along the way forwards toward whichever claim looks closest by its own metrics. No DNS trickery, no client redirect, no extra round trip; the routing fabric itself does the geo-selection for free.

This is exactly how most Domain Name System root servers and major Content Delivery Network operators work: a CDN's edge IP is the same number everywhere on Earth, and a user in Tokyo and a user in Toronto both connect to "the same" address while actually landing on different physical machines, purely because BGP routed them differently. It also gives a natural DDoS mitigation property — attack traffic aimed at one anycast address gets spread across every location advertising it, diluting the impact instead of concentrating it on a single target the way a unicast IP would.

The tradeoff is that anycast routing can be unstable for long-lived connections: BGP paths can change mid-session and silently redirect a client to a different physical server, which is fine for stateless UDP lookups (DNS queries) but awkward for a stateful TCP connection expecting to keep talking to the same backend for its whole lifetime — one reason CDNs still terminate TCP at the edge and manage state carefully behind it.