IPv4 Multicasting

Updated: February 13, 2009

Applies To: Windows 7, Windows Server 2008 R2

Multicasting is the sending of network traffic to a group of endpoints. Only those members in the group of endpoints that are listening for the multicast traffic (the multicast group) process the multicast traffic. All other nodes ignore the multicast traffic.

The concept of group membership is central to IP multicasting. IP multicast datagrams are sent to a group, and only members of the group receive the datagrams. A group is identified by a single IP multicast address, which is an IP address in the Class D range of to ( with a subnet mask of These Class D addresses are known as group addresses. A source host sends multicast datagrams to a group address. Destination hosts inform a local router that they need to join the group.

In an IP multicast-enabled intranet, any host can send IP multicast datagrams to any group address, and any host can receive IP multicast datagrams from any group address regardless of its location. To facilitate this capability, the hosts and routers on the intranet must support IP multicasting. Hosts use the Internet Group Management Protocol (IGMP) for establishing group membership. Routers use multicast routing protocols for forwarding multicast data.

The following figure shows a multicast-enabled intranet.

Multicast-Enabled Network

In this illustration, the hosts and routers are multicast-enabled so that the following can occur:

  • The sending host sends multicast datagrams to a designated group address.

  • Routers forward the multicast datagrams to any network segments that include group members. Routers can forward multicast traffic across a network, between networks, and across the Internet.

  • Receiving hosts inform a local router to join the group, and then they receive all subsequent datagrams sent to the group address.

  • If a receiving host leaves the group and detects that it might be the last group member on the subnet, it can contact the local router to leave the group, informing the router to stop forwarding the multicast datagrams to that subnet.

Multicasting provides an efficient way to support high-bandwidth, one-to-many applications on a network:

  • Multicasting can dramatically reduce network traffic by sending a single copy of the data.

  • Hosts can be configured for multicasting without hardware upgrades.

  • Because newer routers already support multicast forwarding and multicast routing protocols, enabling multicasting on a network is practical and cost-effective.

Multicasting is useful for many types of one-to-many applications, such as the following:

  • Multimedia, such as video conferencing and collaborative computing.

  • Automatic discovery of resources in a network (for example, TCP/IP router discovery uses multicasting by default, and WINS uses multicasting during automatic discovery of replication partners).

  • Datacasting, such as file distribution or database synchronization.

  • Mobile computer support, such as remote address book updating.

  • Distribution of organizational publications.

Windows and RRAS do not include multicast routing protocols, such as Distance Vector Multicast Routing Protocol (DVMRP), Multicast Extensions to Open Shortest Path First (MOSPF), and Protocol Independent Multicast (PIM), although RRAS does support multicast routing protocols developed by independent software vendors (ISVs).

As an alternative, you can use RRAS to forward multicast traffic. In this case, RRAS uses IGMP as an IP routing protocol component. Router interfaces are configured in one of two operating modes: IGMP router mode or IGMP proxy mode. The purpose of IGMP router mode is to forward multicast traffic in a single-router intranet. The purpose of IGMP proxy mode is to connect a single-router intranet to a multicast-capable intranet or the Internet.

Although RRAS uses IGMP in a limited way to enable multicast forwarding on an intranet, it is not the equivalent of a true multicast routing protocol. The RRAS IGMP routing protocol component supports multicast forwarding for several network topologies. For more information about IGMP in RRAS, see Internet Group Management Protocol.

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