Understanding IPv6 anycast: Which statement accurately describes an IPv6 anycast address and how packets to it are delivered?
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APackets addressed to a unicast address are delivered to a single interface.
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BPackets are delivered to all interfaces identified by the address. This is also called one-to-many addresses.
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CThis address identifies multiple interfaces and the anycast packet is only delivered to one address. This address can also be called one-to-one-of-many.
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DThese addresses are meant for nonrouting purposes, but they are almost globally unique so it is unlikely they will have an address overlap.
Answer
Correct Answer: This address identifies multiple interfaces and the anycast packet is only delivered to one address. This address can also be called one-to-one-of-many.
Explanation
Introduction / Context:IPv6 introduces anycast as a native addressing model. While unicast targets a single interface and multicast targets multiple interfaces, anycast provides a special pattern: the same address assigned to multiple interfaces, with delivery to the nearest (according to routing metrics).
Given Data / Assumptions:
- Routers use standard unicast routing to reach the anycast address.
- Multiple devices share the same anycast address.
- Applications may use anycast for services like DNS resolvers or content distribution.
Concept / Approach:
Anycast is best summarized as “one-to-one-of-many.” The network forwards to the topologically closest instance advertising that address. This differs from multicast (one-to-many) and unicast (one-to-one). The behavior is determined by routing, not by special transport signaling.
Step-by-Step Solution:
Assign the same IPv6 anycast address to multiple servers/routers.Announce that prefix in the routing domain.Clients sending to the anycast address are routed to the nearest instance.Verification / Alternative check:
Tracing from different network locations shows traffic reaching different anycast endpoints depending on topology and metrics. No special host configuration beyond using the anycast destination is required.
Why Other Options Are Wrong:
(A) defines unicast, not anycast.
(B) defines multicast delivery semantics.
(D) loosely describes unique-local or special-use addresses, not anycast behavior.
Common Pitfalls:
Confusing anycast with load balancing at Layer 4; assuming packets are duplicated like multicast; forgetting that failure or withdrawal of one instance causes routing to shift to another seamlessly.
Final Answer:
This address identifies multiple interfaces and the anycast packet is only delivered to one address. This address can also be called one-to-one-of-many.