A truly multicast aware system will only allow video data to be transmitted to the ports requesting the data (by use of a hardware or software decoder attached to the network) and will not flood the network. A multicast transmission requires a multicast aware network environment to avoid flooding all ports with the video data being sent. All network hardware (switches, routers, etc.) should be able to detect a multicast destination IP address and forward it appropriately based on IGMP (Internet Group Management Protocol) snooping. IGMP allows a device (PC or hardware decoder) to receive messages and streams addressed to a multicast group, by informing a local router of its membership.
For more info on IGMP please refer to this FAQ.