What is anycast?
Anycast is a technique where one IP address is announced from many locations and traffic reaches the nearest. We cover DNS, CDN and anti-DDoS use cases.
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Anycast is an addressing and routing method in which the same IP address (or prefix) is announced simultaneously from many locations on the internet. Each user's network, following BGP, picks the shortest path and delivers packets to the topologically nearest node announcing that address. From the client's point of view there is a single address, while in reality many distributed servers handle it. Anycast underpins the global DNS, CDNs and modern anti-DDoS protection.
How anycast works
In the anycast model an operator places an identical service in many points of presence (PoPs) and announces the same prefix in BGP from each of them. Routers across the internet treat these announcements as many paths to one destination and, according to their policy and AS_PATH length, choose the best, usually the nearest. When one node stops announcing the prefix (a failure or overload), traffic automatically shifts to the next nearest one, with no address change on the client side.
Where anycast is used
- DNS: root servers and large DNS operators use anycast to answer from the nearest node and survive attacks.
- CDN: content delivery networks steer a user to the nearest edge node, which cuts latency.
- Anti-DDoS protection: attack traffic spreads across many scrubbing locations instead of hitting one point.
- NTP and other infrastructure services that benefit from proximity and resilience.
Anycast versus unicast and its role in DDoS defense
In classic unicast one address maps to one server in one location, so all traffic (including an attack) lands in the same place. Anycast disperses that traffic geographically. A volumetric attack is absorbed and split across many nodes, each of which only has to handle a portion of the traffic close to its source. That makes anycast one of the most effective ways to add capacity and resilience to internet-facing services.
Anycast and AS202520 SkyPass
Deploying anycast requires your own independent address space and an ASN so you can announce the same prefix from many locations over BGP. AS202520 SkyPass, as an operator with its own network and wide peering in Poland, provides stable, low-latency BGP connectivity that is a solid foundation for anycast services aimed at Polish and European users.
Frequently asked questions
How is anycast different from unicast?
In unicast one IP address points to one server in one location. In anycast the same address is announced from many places, and routing delivers traffic to the nearest of them.
Does anycast help defend against DDoS?
Yes. Anycast spreads attack traffic across many locations, so no single node takes the whole volume. It is one of the main mechanisms used by scrubbing networks.
Do I need my own ASN and addresses for anycast?
Yes. To announce the same prefix from many locations over BGP you need your own provider-independent address space and an ASN.
Does anycast work for TCP?
Yes, in practice it works well because the path to the nearest node is usually stable during a session. On rare route changes a session may be steered to another node and re-established.