Traditional IP communication allows a host to send packets to a single host (unicast transmission) or to all hosts (broadcast transmission). IP multicast provides a third possibility: allowing a host to send packets to a subset of all hosts as a group transmission. This overview provides a brief, summary overview of IP Multicast. First, general topics such as multicast group concept, IP multicast addresses, and Layer 2 multicast addresses are discussed. Then intradomain multicast protocols are reviewed, such as Internet Group Management Protocol (IGMP), Cisco Group Management Protocol (CGMP), Protocol Independent Multicast (PIM) and Pragmatic General Multicast (PGM). Finally, interdomain protocols are covered, such as Multiprotocol Border Gateway Protocol (MBGP), Multicast Source Directory Protocol (MSDP), and Source Specific Multicast (SSM).

This document is intended as a general "refresher" on IP multicast, not a tutorial. It is assumed that the reader is familiar with TCP/IP, Border Gateway Protocol (BGP), and networking in general. Please refer to Beau Williamson's book titled Developing IP Multicast Networks, Volume 1 (Cisco Press, 1999) if you need more information about any of the topics presented in this overview.