What is Multicast?

Multicast is a communication method in IP networking that sends data from one source to multiple destinations simultaneously. Unlike unicast, which sends separate copies of each packet to every receiver, multicast allows the network infrastructure to replicate packets only at points where paths diverge. This makes it efficient for applications that need to deliver the same content to many hosts, such as live video streaming, audio conferencing, software distribution, or real-time financial data feeds.

Multicast operates at the network layer using IP multicast addresses. IPv4 uses the Class D address range 224.0.0.0 to 239.255.255.255, while IPv6 uses the ff00::/8 prefix. Hosts join a multicast group by signaling their interest via the Internet Group Management Protocol (IGMP) for IPv4 or Multicast Listener Discovery (MLD) for IPv6. Routers then use a multicast routing protocol, such as Protocol Independent Multicast (PIM), to build distribution trees that efficiently forward traffic only to network segments containing group members. Reverse Path Forwarding (RPF) checks prevent loops.

Multicast sits between unicast and broadcast on the delivery spectrum. It is more scalable than broadcast because it does not flood traffic to all hosts, and more efficient than unicast when serving many receivers. However, it requires routers and switches to maintain group state, which can be complex in large or dynamic networks. Multicast is often used in enterprise networks, content delivery networks, and IPTV systems. It is not typically enabled by default on the public internet due to operational challenges, so many real-world deployments use overlay solutions like Application Layer Multicast or multicast VPNs.