Multicast, Anycast, and Broadcast Explained
Learn the three methods of one-to-many communication in IP networks: multicast, anycast, and broadcast.
Four Ways to Address IP Packets
IP communication can be classified by how many recipients receive a packet:
| Method | Recipients | Scope |
|---|---|---|
| Unicast | One specific device | Global |
| Broadcast | All devices on a subnet | Local subnet only |
| Multicast | A group of subscribed devices | Local or global |
| Anycast | Nearest device sharing an address | Global |
Most internet traffic is unicast (one sender, one receiver). The other three methods serve specialized purposes.
Broadcast
Broadcast sends a packet to every device on the local network segment. The destination address 255.255.255.255 (limited broadcast) or the subnet broadcast address (e.g., 192.168.1.255 for a /24) triggers delivery to all hosts.
Common uses: - DHCP discovery -- A device broadcasts to find available DHCP servers. - ARP requests -- "Who has IP 192.168.1.1? Tell 192.168.1.50." - NetBIOS name resolution -- Legacy Windows name lookups.
Broadcasts do not cross router boundaries, which limits their scope to the local subnet. IPv6 eliminates broadcast entirely, replacing it with multicast.
Multicast
Multicast delivers packets to a specific group of interested receivers. Devices join a multicast group by subscribing to a multicast IP address in the range 224.0.0.0 to 239.255.255.255 (Class D).
Sender -> Multicast group 239.1.1.1
-> Receiver A (subscribed) - receives
-> Receiver B (subscribed) - receives
-> Receiver C (not subscribed) - ignores
Multicast uses the IGMP (Internet Group Management Protocol) for group membership and relies on multicast-capable routers to forward traffic only where subscribers exist.
Real-world applications: - IPTV -- Television streams delivered to thousands of viewers without duplicating traffic for each one. - Stock market feeds -- Financial data multicast to trading systems. - Video conferencing -- Efficient one-to-many video distribution within corporate networks.
Anycast
Anycast assigns the same IP address to multiple servers in different locations. When a client sends a packet to an anycast address, network routing delivers it to the nearest server (by BGP path metrics).
Client in Tokyo -> 1.1.1.1 -> Cloudflare Tokyo PoP
Client in London -> 1.1.1.1 -> Cloudflare London PoP
Anycast is widely used for: - DNS root servers -- Each of the 13 root server letters is served by dozens of anycast instances worldwide. - CDN edge nodes -- Cloudflare, Google, and Akamai use anycast to route users to the closest data center. - DDoS mitigation -- Attack traffic is distributed across all anycast locations, preventing any single site from being overwhelmed.
Choosing the Right Method
- Use unicast for direct, one-to-one communication.
- Use multicast when the same data must reach many recipients efficiently (especially on LANs).
- Use anycast for global services that need low latency and high availability.
- Broadcast is legacy -- avoid it in new designs and prefer multicast or unicast.