408 PART III Routing and Routing Protocols The Ethernet frame destination address is in a special form, starting with 01 and ending in 02:02:02 --which corresponds to the 239.2.2.2 multicast group address. We'll explore the rules for determining this frame address in material following. Note that the packet is addressed to the entire group, not an individual host (as in unicast). How does the network know where to send replicated packets? Two strategies (dis- cussed later in the chapter) are to send content everywhere and then stop if no one says they are listening (flood-and-prune, or dense mode), or to send content only to hosts that have indicated a desire to receive the content (sparse mode). The figure also shows that the Windows XP receiver ( 10.10.12.222 ) is generating IGMPv3 membership reports sent to multicast group address 224.0.0.22 (the IGMP multicast group). XP does this to keep the multicast content coming, even though the socket sender program has no idea what it means. These messages from XP to the IGMP group sometimes cause consternation with Windows network administrators, who are not always familiar with multicast and wonder where the 224.0.0.22 "server" could be. Now let's set our multicast group send program to span the router network from LAN1 to LAN2. We'll start the socket utility sending on wincli1 (10.10.11.51) , using multicast group 239.1.1.1 and UDP port 11111 . The listener will still be wincli2