This memo specifies the extensions required for a host implementation to support IP multicasting in the ARPA-Internet. It supersedes RFC-966 and is a proposed protocol standard for IP multicasting. The document outlines the status of the memo, introduces the concept of IP multicasting, and defines the levels of conformance (Level 0, Level 1, and Level 2). It details the responsibilities of multicast agents and the creation and maintenance of host groups. The memo also specifies the extensions needed for host IP implementations to support IP multicasting, including sending and receiving multicast IP datagrams and managing group membership. It provides a layered model for the extensions and discusses the extensions to the IP service interface, IP module, local network service interface, and Ethernet local network module. The document includes an appendix on the Internet Group Management Protocol (IGMP) and another on issues related to host group addresses.This memo specifies the extensions required for a host implementation to support IP multicasting in the ARPA-Internet. It supersedes RFC-966 and is a proposed protocol standard for IP multicasting. The document outlines the status of the memo, introduces the concept of IP multicasting, and defines the levels of conformance (Level 0, Level 1, and Level 2). It details the responsibilities of multicast agents and the creation and maintenance of host groups. The memo also specifies the extensions needed for host IP implementations to support IP multicasting, including sending and receiving multicast IP datagrams and managing group membership. It provides a layered model for the extensions and discusses the extensions to the IP service interface, IP module, local network service interface, and Ethernet local network module. The document includes an appendix on the Internet Group Management Protocol (IGMP) and another on issues related to host group addresses.