Multicast

Multicast vs. Alternatives

To stream video to 10,000 viewers, UnicastA network communication method that sends data from one sender to one specific receiver. The most common form of IP communication, used for web browsing, email, and file transfers. requires 10,000 separate streams — one per viewer — multiplying bandwidth at the source by 10,000. BroadcastA network communication method that sends data to all devices on a subnet simultaneously. The broadcast address is the last address in a subnet, such as 192.168.1.255 for a /24 network. sends to everyone on the subnet, including uninterested devices. Multicast sends a single stream that the network replicates only where needed: at the last router before branches separate toward interested receivers. This is orders of magnitude more efficient for one-to-many distribution.

The Multicast Address Space

IPv4 multicast addresses occupy 224.0.0.0/4 (224.0.0.0 through 239.255.255.255). This range is further subdivided: 224.0.0.0/24 is reserved for link-local use (OSPF, PIM, IGMP use addresses here); 232.0.0.0/8 is for source-specific multicast; 239.0.0.0/8 is the administratively scoped "private" multicast space, analogous to Private IP AddressAn IP address from reserved ranges (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16) used within local networks. Private addresses are not routable on the public internet and require NAT for external communication..

IPv6Internet Protocol version 6. The successor to IPv4 using 128-bit addresses (e.g., 2001:0db8::1), providing a virtually unlimited address space of 3.4 x 10^38 addresses. Designed to solve IPv4 address exhaustion. encodes multicast in the ff00::/8 prefix. Every IPv6Internet Protocol version 6. The successor to IPv4 using 128-bit addresses (e.g., 2001:0db8::1), providing a virtually unlimited address space of 3.4 x 10^38 addresses. Designed to solve IPv4 address exhaustion. device automatically joins several multicast groups — including the all-nodes group (ff02::1) and its own solicited-node group — replacing the BroadcastA network communication method that sends data to all devices on a subnet simultaneously. The broadcast address is the last address in a subnet, such as 192.168.1.255 for a /24 network. functions that IPv4 relies on.

IGMP and Multicast Routing

Receivers join multicast groups using IGMP (Internet Group Management Protocol) on IPv4Internet Protocol version 4. The fourth revision of IP using 32-bit addresses (e.g., 192.168.1.1), providing approximately 4.3 billion unique addresses. Still the most widely used internet protocol despite address exhaustion. or MLD on IPv6Internet Protocol version 6. The successor to IPv4 using 128-bit addresses (e.g., 2001:0db8::1), providing a virtually unlimited address space of 3.4 x 10^38 addresses. Designed to solve IPv4 address exhaustion.. A router running IGMP snooping tracks which ports have interested receivers and forwards multicast traffic only to those ports — preventing unnecessary flooding.

Multicast routing protocols (PIM-SM, PIM-SSM, MOSPF) build distribution trees across routers so that multicast traffic travels efficient paths from source to receivers across multiple subnets, without the source needing to know who is watching.