This memo specifies the extensions required of a host implementation of the Internet Protocol (IP) to support internetwork multicasting. It supersedes the specification in RFC-966 and constitutes a proposed protocol standard for IP multicasting in the ARPA-Internet. The memo outlines the mechanisms for IP multicasting, including the dynamic membership of host groups, the use of class D IP addresses for group identification, and the role of multicast agents in managing group membership and delivery. IP multicasting allows a datagram to be sent to a group of hosts identified by a single IP destination address. The datagram is delivered to all members of the group with the same reliability as unicast datagrams. Host groups can be permanent or transient. Permanent groups have well-known, administratively assigned IP addresses, while transient groups are dynamically assigned when a group is created. Multicast agents are responsible for creating and maintaining group memberships and delivering multicast datagrams across networks. Hosts can request the creation of new groups and join or leave existing groups by exchanging messages with neighboring agents. The memo specifies the required extensions for host IP implementations to support IP multicasting, including sending and receiving multicast datagrams and managing group membership. The IP service interface is extended to support these operations, and the IP module is modified to recognize host group addresses. The local network service interface is also extended to handle multicast datagrams. For Ethernet networks, IP host group addresses are mapped to Ethernet multicast addresses. Other networks may use similar mapping techniques. The Internet Group Management Protocol (IGMP) is used to manage group membership. It allows hosts to join, leave, or confirm their membership in a group. IGMP messages are used to communicate between hosts and multicast agents. The memo also discusses the use of host group addresses as logical addresses and the recycling of transient group addresses to ensure availability and robustness. The memo outlines three levels of conformance: Level 0 (no support for IP multicasting), Level 1 (support for sending but not receiving multicast datagrams), and Level 2 (full support for IP multicasting). The memo also provides details on the structure of host group addresses, the model of a host IP implementation, and the protocols used for sending and receiving multicast datagrams.This memo specifies the extensions required of a host implementation of the Internet Protocol (IP) to support internetwork multicasting. It supersedes the specification in RFC-966 and constitutes a proposed protocol standard for IP multicasting in the ARPA-Internet. The memo outlines the mechanisms for IP multicasting, including the dynamic membership of host groups, the use of class D IP addresses for group identification, and the role of multicast agents in managing group membership and delivery. IP multicasting allows a datagram to be sent to a group of hosts identified by a single IP destination address. The datagram is delivered to all members of the group with the same reliability as unicast datagrams. Host groups can be permanent or transient. Permanent groups have well-known, administratively assigned IP addresses, while transient groups are dynamically assigned when a group is created. Multicast agents are responsible for creating and maintaining group memberships and delivering multicast datagrams across networks. Hosts can request the creation of new groups and join or leave existing groups by exchanging messages with neighboring agents. The memo specifies the required extensions for host IP implementations to support IP multicasting, including sending and receiving multicast datagrams and managing group membership. The IP service interface is extended to support these operations, and the IP module is modified to recognize host group addresses. The local network service interface is also extended to handle multicast datagrams. For Ethernet networks, IP host group addresses are mapped to Ethernet multicast addresses. Other networks may use similar mapping techniques. The Internet Group Management Protocol (IGMP) is used to manage group membership. It allows hosts to join, leave, or confirm their membership in a group. IGMP messages are used to communicate between hosts and multicast agents. The memo also discusses the use of host group addresses as logical addresses and the recycling of transient group addresses to ensure availability and robustness. The memo outlines three levels of conformance: Level 0 (no support for IP multicasting), Level 1 (support for sending but not receiving multicast datagrams), and Level 2 (full support for IP multicasting). The memo also provides details on the structure of host group addresses, the model of a host IP implementation, and the protocols used for sending and receiving multicast datagrams.