Multicast sends one packet to multiple receivers simultaneously — more efficient than unicast (separate copy per receiver) for one-to-many applications: video streaming, financial data distribution.
Multicast addresses: 224.0.0.0/4. IGMP: hosts join/leave multicast groups. PIM: multicast routing protocol. Multicast is more complex to configure and troubleshoot than unicast. Security: unauthenticated multicast group joins — filter with IGMP snooping on switches.