Network Addressing Architectures: Unicast, Multicast, and Anycast

If you’ve ever wondered how a data packet actually knows where to go, you have to look at the addressing methodology. It isn’t just about having an IP address; it’s about how the network announces that address to the world.

Most people assume the internet is strictly a point-to-point system. You click a link, your computer calls a server, the server answers. That’s true 90% of the time. But for the internet to handle live video streams or survive massive DDoS attacks, we need more sophisticated ways to route traffic.

Here is a breakdown of the three primary routing schemes: Unicast, Multicast, and Anycast.

1. Unicast: The One-to-One Conversation

This is the bread and butter of the internet. If the internet were a postal service, Unicast is a standard letter. You write one destination address on the envelope, and it goes to exactly one house.

The History

Unicast is as old as the internet itself. When ARPANET was established in the late 1960s, the fundamental concept was connecting one host to another. IPv4, and later IPv6, were built with this as the default behavior. If you assign an IP address to a network interface card (NIC), you are establishing a Unicast identity.

How It Works

• Source: One transmitter.
• Destination: One receiver.
• Mechanism: The router looks at the destination IP, checks its routing table for the best path, and forwards the packet.

Use Cases

Almost everything you do daily relies on this. * Web browsing (HTTP/HTTPS). * Sending an email. * Downloading a file via FTP. * SSH sessions.

Pros and Cons

The Good: * Simplicity: It’s easy to implement. Every router understands it. * Reliability: Great for TCP connections where flow control and error correction are needed between two specific endpoints.

The Bad: * Scaling inefficiency: If 1,000 people want to watch the same live 4K video stream via Unicast, the server has to send 1,000 separate copies of the data. This crushes bandwidth.

2. Multicast: The Efficient Broadcaster

If Unicast is a letter, Multicast is a town hall meeting. The speaker talks once, and everyone who chose to step into the room hears it.

The History

Multicast roots go back to the 1980s. Steve Deering defined IP Multicast in his PhD dissertation, leading to the creation of the MBONE (Multicast Backbone) in the early 90s. The goal was ambitious: enable true internet radio and TV without melting down the infrastructure.

How It Works

• Source: One transmitter.
• Destination: A specific group of receivers (subscribed hosts).
• Mechanism: The server sends a single packet addressed to a reserved Multicast IP range (e.g., 224.0.0.0 to 239.255.255.255). Routers replicate the packet only when necessary to reach the downstream subscribers.

Use Cases

Multicast shines where simultaneous data consumption happens. * IPTV: When you change channels on a cable box running over IP, you are sending an IGMP join request to a new multicast group. * Financial Markets: Stock tickers need to hit thousands of trader terminals at the exact same millisecond. * System Deployment: Imaging 500 corporate laptops simultaneously over a LAN.

Pros and Cons

The Good: * Bandwidth efficiency: The sender uses the same amount of bandwidth regardless of whether there are 5 clients or 5 million. * CPU Load: Drastically reduces the load on the transmitting server.

The Bad: * Routing complexity: It relies on protocols like PIM (Protocol Independent Multicast) and IGMP. * The “Public Internet” problem: Most ISPs block multicast traffic at their borders. It’s too hard to bill for, and too easy to use for DDoS amplification. Consequently, Multicast basically only exists inside private networks (LANs/VPNs) or carrier backbones, not across the open web.

3. Anycast: The “Closest One Wins”

Anycast is the most confusing concept for beginners because it breaks the mental model of “One IP = One Server.” In Anycast, the same IP address exists in dozens of places around the world at the same time.

The History

First described in RFC 1546 in 1993, Anycast was initially theoretical. It wasn’t a protocol update; it was a clever manipulation of BGP (Border Gateway Protocol). It gained massive popularity in the 2000s as the need for global DNS root servers and CDNs (Content Delivery Networks) exploded.

How It Works

• Source: One transmitter (the client).
• Destination: The “nearest” receiver within a group sharing the same IP.
• Mechanism: You announce the same IP prefix from New York, London, and Tokyo. When a user in Paris sends a packet to that IP, the internet’s routing logic (BGP) naturally guides the packet to the London server because it is the shortest path (topologically).

Use Cases

• DNS: Google’s 8.8.8.8 or Cloudflare’s 1.1.1.1. These aren’t single computers; they are clusters of thousands of servers globally, all announcing the same IP.
• CDNs: Getting content to load fast by connecting you to a server in your city.
• DDoS Mitigation: If an attacker in Russia tries to flood a US-based service, Anycast isolates the attack. The traffic gets routed to the “nearest” scrubbing center in Europe, leaving the US infrastructure untouched.

Pros and Cons

The Good: * Resilience: If the London server dies, the route is withdrawn, and traffic automatically shifts to the next closest site (maybe Frankfurt). * Speed: Users connect to the lowest-latency node available.

The Bad: * Troubleshooting is a nightmare: Since the traffic goes to different physical locations based on where the user is standing, debugging connection errors is incredibly difficult. * Statefulness issues: Standard TCP connections can break if routing topology changes mid-stream and your packets suddenly get routed to a different server that doesn’t know who you are.

Summary Comparison

Final Thoughts

If you are building a standard web app, you stick to Unicast. It works. It’s simple.

If you are building a trading floor or a private video distribution network, Multicast is your best friend.

But if you are tasked with keeping a service online globally, fighting off botnets, or serving DNS, you have to engineer Anycast into your infrastructure.