3com DUA1750-2BAA01 Manual De Usuario

3Com Switch 8800 Configuration Guide 

Chapter 23  IP Multicast Overview

 

23-4 

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Occasional communication for training and cooperation 

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Data storage and finance (stock) operation 

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Point-to-multipoint data distribution 

With the increasing popularity of multimedia services over IP network, multicast is 

gaining its marketplace. In addition, the multicast service becomes popular and 

prevalent gradually. 

In multicast mode, there are questions about where to send the information, how to 

locate the destination or know the receiver. All these questions can be narrowed down 

to multicast addressing. To guarantee the communication between a multicast source 

and a multicast group (that is, a group of receivers), the network layer multicast 

address (namely the IP multicast address) is required, along with the technique to 

correlate it with the link layer MAC multicast address. Following is the introduction to 

these two kinds of addresses.  

According to the definition in Internet Assigned Number Authority (IANA), IP addresses 

fall into four types: Class A, Class B, Class C and Class D. Unicast packets use IP 

addresses of Class A, Class B or Class C, depending on specific packet scales. 

Multicast packets use IP addresses of Class D as their destination addresses, but  

Class D IP addresses cannot be contained in the source IP field of IP packets.  

During unicast data transmission, a packet is transmitted "hop-by-hop" from the source 

address to the destination address. However, in IP multicast environment, a packet has 

more than one destination address, or a group of addresses. All the information 

receivers are added to a group. Once a receiver joins the group, the data for this group 

address starts flowing to this receiver. All members in the group can receive the 

packets. This group is a multicast group. 

Membership here is dynamic, and a host can join or leave the group at any time. A 

multicast group can be permanent or temporary. Some multicast group addresses are 

allocated by IANA, and the multicast group is called permanent multicast group. The IP 

addresses of a permanent multicast group are unchangeable, but its membership is 

changeable, and the number of members is arbitrary. It is quite possible for a 

permanent group to not a single member. Those not reserved for permanent multicast 

groups can be used by temporary multicast groups. Class D multicast addresses range 

from 224.0.0.0 to 239.255.255.255. More information is listed in Table 23-1 Ranges