Anycast is a routing method where the same IP address is announced from multiple physical locations via BGP. Each client's traffic goes to whichever announcing site the routing system considers closest, usually by network path rather than geography.
Why it matters
Anycast is how large DNS providers, root servers, public resolvers like 1.1.1.1 and many CDNs serve a single address worldwide. Users get answers from nearby sites, which lowers latency, and a failed site's route is withdrawn so traffic shifts to the others. Because an attack is spread over every site, it also helps absorb DDoS floods.
Example
You can see anycast at work by tracing the same address from two networks:
$ traceroute 1.1.1.1Two users in different countries querying 1.1.1.1 reach different data centers, even though the IP is identical. An ASN lookup shows the operator (for 1.1.1.1, AS13335).
Common mistakes
- Assuming "closest" always means fastest. BGP follows policy and cost, and sometimes picks a distant site.
- Using anycast for long-lived stateful TCP sessions without understanding that a route change can move a user mid-connection.
- Mixing up anycast with GeoDNS, which returns different IPs based on the resolver's location.
- Expecting one traceroute to represent every user.
Related
- CDN
- Nameserver
- DDoS
- गाइड: CDN explained
- टूल: ASN lookup
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