Anycast: one IP address from multiple network locations
With Anycast, the same IP prefix is advertised from multiple network locations. Routing selects an available path according to BGP policy, so users in different networks can reach different servers while using the same destination address.
Anycast is routing, not a URL redirect
Several sites originate the same prefix and routers select among the available routes. “Nearest” usually means preferable according to routing policy; it does not necessarily mean the shortest geographic distance.
DNS services and CDNs commonly use this design. It can shorten paths and provide resilience, but it cannot guarantee identical latency for every user.
Why diagnostics can look surprising
After a BGP policy or topology change, the same IP can lead to another site. Consequently, IP location information may describe the network or registered location rather than the particular edge server that answered.
Compare paths from different networks using the traceroute guide. Different routes to the same Anycast address are expected.
What Anycast does not solve
Anycast does not replace application load balancing, state synchronisation or origin security. A configuration fault can also affect several sites at once. Operational guidance for DNS Anycast is documented in RFC 4786.
Locate the fault by network layer
A useful workflow starts at the edge of the local network. Check the interface address, default route and DNS configuration, then test the local gateway before moving to the public address and remote service. This prevents a Wi-Fi or Ethernet problem from being mistaken for an ISP routing problem. If the gateway remains stable while latency appears farther away, compare several destinations and repeat the measurement at another time.
Intermediate devices are not required to answer every diagnostic probe. Control-plane filtering, rate limiting and asymmetric routing can change what a trace displays while the application path continues to carry traffic normally.
Addresses, routes and the network you observe
A device's local address, its public egress address and the destination address describe different parts of the path. NAT, CGNAT, VPNs, proxies, CDNs and Anycast introduce additional boundaries, so one IP address should not automatically be treated as an identity for a user or a physical server. Record the destination and time of a test because DNS and BGP can lead a later measurement to different infrastructure.
With IPv6, one interface can also have several addresses, including temporary addresses. Source-address selection is performed by the network stack and can affect the path that is observed.
Make measurements reproducible
A single test is a snapshot. Repeat measurements in a series and record whether the connection is wired or wireless, whether a VPN is active, which resolver is in use and whether background traffic is present. Compare latency to the same destination and run throughput tests under comparable conditions. If latency rises only while the link is busy, investigate queueing and bufferbloat separately.
Keep the context as well as the final number. That makes it possible to distinguish a persistent configuration fault from a brief route change, radio congestion or a different remote edge being selected.