multicast

Multicast is a method of network communication in which data is delivered from one or more sources to multiple receivers that have explicitly joined a multicast group. It enables one-to-many delivery, allowing a single stream to be efficiently distributed to many recipients without creating separate copies for each.

In IP multicast, receivers indicate interest by joining a multicast group identified by a special IP address. Routers construct distribution trees to forward traffic from sources to all active group members, using multicast routing protocols to manage replication and forwarding across the network.

Addressing and scope differ by protocol version. IPv4 multicast uses addresses in 224.0.0.0/4, with 239.0.0.0/8 reserved for administratively scoped groups. IPv6 uses the FF00::/8 address space, with scope identifiers to limit reachability. Membership is managed with protocols such as IGMP (IPv4) and MLD (IPv6), while multicast routing relies on protocols like Protocol Independent Multicast (PIM) in its sparse, dense, or sparse-dense variants to build and maintain delivery trees.

Multicast traffic is generally best-effort and does not provide inherent end-to-end reliability, sequencing, or flow control. Applications commonly run over UDP and may implement their own error handling or forward error correction as needed.

Common uses include television-quality streaming (IPTV), video conferencing, real-time data feeds, and large-scale software updates within enterprise networks. Limitations include the need for multicast-enabled infrastructure, management of group membership and access control, security concerns, and limited deployment on the public Internet.