Anycast vs Unicast vs Multicast

Anycast vs Unicast vs Multicast

Why This Matters

Think of network addressing like different ways to send mail. Unicast is like sending a letter to one person (one-to-one). Multicast is like sending a newsletter to a mailing list (one-to-many). Anycast is like sending mail to "the nearest post office"—it goes to the closest one (one-to-nearest). Each serves different purposes: unicast for direct communication, multicast for broadcasting to groups, anycast for routing to nearest server.

This matters because the choice of addressing affects performance, scalability, and network efficiency. Unicast is simple but doesn't scale for broadcasting (sending to many requires many packets). Multicast is efficient for one-to-many (one packet reaches many). Anycast is useful for load balancing and redundancy (route to nearest server). Understanding this helps you design efficient networks.

In interviews, when someone asks "How would you design a content delivery system?", they're testing whether you understand anycast. Do you know when to use unicast vs multicast vs anycast? Most engineers don't. They just use unicast and wonder why it doesn't scale.

What Engineers Usually Get Wrong

Most engineers think "unicast, multicast, and anycast are just different ways to send data." But they have different use cases and network requirements. Multicast requires multicast routing (not all networks support it). Anycast requires special routing configuration. Understanding this helps you choose the right addressing method.

Engineers also don't understand that anycast is used for DNS and CDNs. DNS root servers use anycast—the same IP address is advertised from multiple locations, and routing sends queries to the nearest server. This provides load balancing and redundancy. Understanding this helps you understand how DNS and CDNs work.

How This Breaks Systems in the Real World

A service was broadcasting updates to 10,000 clients using unicast. Each update required 10,000 packets (one per client). The network was overwhelmed. The fix? Use multicast. Send one packet, and it reaches all clients. This reduces network traffic by 10,000x. But multicast requires multicast routing, which not all networks support.

Another story: A service was using anycast for DNS but didn't configure it correctly. Queries were routed to distant servers instead of nearby ones, increasing latency. The fix? Configure anycast correctly. Ensure routing advertisements are correct, and servers are distributed geographically. This ensures queries reach the nearest server.

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Examples

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Common Pitfalls

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Interview Questions

Beginner

Q: [Beginner-level question about the topic]

A: [Comprehensive answer with explanation]

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Intermediate

Q: [Intermediate-level question with practical scenario]

A: [Detailed answer with implementation details]

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Senior

Q: [Senior-level system design or complex scenario question]

A: [In-depth answer with architecture, trade-offs, and best practices]

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• Routing Protocols (OSPF/BGP) - Routing protocols determine how traffic is routed, understanding routing types explains protocol behavior

• CDN Routing - CDNs use anycast for routing, understanding routing types explains CDN architecture

• DNS Resolution Flow - DNS uses anycast for root servers, understanding routing types explains DNS architecture

• OSI Model (7 Layers) - Routing types operate at Layer 3 (Network), understanding the OSI model provides context

• IP Addressing (IPv4/IPv6) - Routing types work with IP addresses, understanding IP addressing explains routing behavior

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• [Key point 3: Practical application]

• [Key point 4: Common use cases]

• [Key point 5: Best practices]