Multicast
Multicast is one-to-many communication where a single packet is delivered to all members of a group simultaneously. The network replicates packets at branch points rather than the sender duplicating them. Used for IPTV, stock market feeds, and cluster discovery. Requires IGMP (hosts) and PIM (routers).
Definition
Multicast delivers packets to multiple receivers without the sender transmitting multiple copies. Hosts join multicast groups (224.0.0.0/4 in IPv4) using IGMP (Internet Group Management Protocol). Routers running PIM (Protocol Independent Multicast) build distribution trees that replicate packets only where needed. A live video stream sent once by the source reaches thousands of viewers without multiplying the origin's bandwidth usage. Multicast is essential for: IPTV delivery (each channel is a multicast group), financial market data feeds (one source, thousands of trading terminals), cluster membership (Kubernetes uses 224.0.0.18 for VRRP), and service discovery (mDNS uses 224.0.0.251). Public internet routers do not forward multicast – it works only within networks that explicitly enable it or via overlay solutions.
Examples
- –ip maddr show – list multicast group memberships on Linux
- –224.0.0.251 – mDNS multicast group (Bonjour/Avahi)
- –iperf -s -u -B 239.1.1.1 – multicast server on group 239.1.1.1
Related Protocols
Related Terms
Unicast
Unicast is one-to-one communication where a packet is sent from one source to one specific destination. Most internet traffic is unicast – HTTP requests, SSH sessions, email delivery. Unicast requires the sender to know the destination IP address. Contrast with multicast (one-to-many) and broadcast (one-to-all).
Broadcast
Broadcast sends a packet to all hosts on a network segment simultaneously. The IPv4 broadcast address (e.g., 192.168.1.255 for /24) delivers to every host on the subnet. ARP, DHCP discovery, and NetBIOS use broadcast. Excessive broadcasts cause broadcast storms. IPv6 eliminated broadcast entirely in favor of multicast.