.0 and 239.255.255.255 are reserved for multicast.) Advantages of Multicast Multicast is scalable because data traverses each link only once (Fig- ure 3-8). Multicast routers will duplicate IP packets only if necessary and even then will duplicate as close to the members of the multicast group as possible. Figure 3-8 shows the same configuration as Figure 3-7. Packets from 131.160.1.112 to 138.85.27.10 and from 131.160.1.112 to 153.88.251.19 are also sent, but, this time, via multicast routing. Links from 131.160.1.112 to R carry the same data just once. R will duplicate the data in order to send it to 138.85.27.10 and 153.88.251.19. This example illustrates two main savings: Half of the bandwidth used from 131.160.1.112 to R is saved. The end systems do not need to know who the members of the multicast group are in order to send data. Senders send data to the multicast address without knowing the identity of all the receivers. Only the routers closest to the receivers know which hosts are members of the multicast group. No central server is keeping track of multicast groups; all the information is distributed among multi- cast routers. The most a multicast router needs to know is whether at least one mem- ber of the multicast group can be reached using one of the router's inter- faces. If the answer to that question is yes, the data will be transmitted through this interface. The Internet Multimedia Conferencing Architecture Figure 3-8 Multicast routing. 131.160.1.112 153.88.251.19 Destination IP address: 224.2.17.12 ROUTER ROUTER ROUTER ROUTER ROUTER ROUTER ROUTER 138.85.27.10 Multicast Routing Protocols Multicast routers use multicast routing protocols to build distribution trees from senders to receivers. These protocols can be grouped into two cate- gories, sparse mode and dense mode. These two modes use different algo- rithms to build trees. Dense Mode Multicast Routing Protocols Dense mode multicast routing protocols work well in networks where most of the hosts are mem- bers of the multicast group. When t