1-6Multicast AddressAs receivers are multiple hosts in a multicast group, you should be concerned about the followingquestions:z What destination should the information source send the information to in the multicast mode?z How to select the destination address?These questions are about multicast addressing. To enable the communication between the informationsource and members of a multicast group (a group of information receivers), network-layer multicastaddresses, namely, IP multicast addresses must be provided. In addition, a technology must beavailable to map IP multicast addresses to link-layer MAC multicast addresses. The following sectionsdescribe these two types of multicast addresses:IP multicast addressInternet Assigned Numbers Authority (IANA) categorizes IP addresses into five classes: A, B, C, D, andE. Unicast packets use IP addresses of Class A, B, and C based on network scales. Class D IPaddresses are used as destination addresses of multicast packets. Class D address must not appear inthe IP address field of a source IP address of IP packets. Class E IP addresses are reserved for futureuse.In unicast data transport, a data packet is transported hop by hop from the source address to thedestination address. In an IP multicast environment, there are a group of destination addresses (calledgroup address), rather than one address. All the receivers join a group. Once they join the group, thedata sent to this group of addresses starts to be transported to the receivers. All the members in thisgroup can receive the data packets. This group is a multicast group.A multicast group has the following characteristics:z The membership of a group is dynamic. A host can join and leave a multicast group at any time.z A multicast group can be either permanent or temporary.z A multicast group whose addresses are assigned by IANA is a permanent multicast group. It is alsocalled reserved multicast group.Note that:z The IP addresses of a permanent multicast group keep unchanged, while the members of thegroup can be changed.z There can be any number of, or even zero, members in a permanent multicast group.z Those IP multicast addresses not assigned to permanent multicast groups can be used bytemporary multicast groups.Class D IP addresses range from 224.0.0.0 to 239.255.255.255. For details, see Table 1-2.Table 1-2 Range and description of Class D IP addressesClass D address range Description224.0.0.0 to 224.0.0.255Reserved multicast addresses (IP addresses forpermanent multicast groups). The IP address224.0.0.0 is reserved. Other IP addresses can be usedby routing protocols.224.0.1.0 to 231.255.255.255233.0.0.0 to 238.255.255.255Available any-source multicast (ASM) multicastaddresses (IP addresses for temporary groups). Theyare valid for the entire network.
