3Com 4200G 12-Port Configuration Manual page 451

As receivers are multiple hosts in a multicast group, you should be concerned about the following
questions:
What destination should the information source send the information to in the multicast mode?
How to select the destination address?
These questions are about multicast addressing. To enable the communication between the information
source and members of a multicast group (a group of information receivers), network-layer multicast
addresses, namely, IP multicast addresses must be provided. In addition, a technology must be
available to map IP multicast addresses to link-layer MAC multicast addresses. The following sections
describe these two types of multicast addresses:
IP multicast address
Internet Assigned Numbers Authority (IANA) categorizes IP addresses into five classes: A, B, C, D, and
E. Unicast packets use IP addresses of Class A, B, and C based on network scales. Class D IP
addresses are used as destination addresses of multicast packets. Class D address must not appear in
the IP address field of a source IP address of IP packets. Class E IP addresses are reserved for future
use.
In unicast data transport, a data packet is transported hop by hop from the source address to the
destination address. In an IP multicast environment, there are a group of destination addresses (called
group address), rather than one address. All the receivers join a group. Once they join the group, the
data sent to this group of addresses starts to be transported to the receivers. All the members in this
group can receive the data packets. This group is a multicast group.
A multicast group has the following characteristics:
The membership of a group is dynamic. A host can join and leave a multicast group at any time.
A multicast group can be either permanent or temporary.
A multicast group whose addresses are assigned by IANA is a permanent multicast group. It is also
called reserved multicast group.
Note that:
The IP addresses of a permanent multicast group keep unchanged, while the members of the
group can be changed.
There can be any number of, or even zero, members in a permanent multicast group.
Those IP multicast addresses not assigned to permanent multicast groups can be used by
temporary multicast groups.
Class D IP addresses range from 224.0.0.0 to 239.255.255.255. For details, see
Table 1-2 Range and description of Class D IP addresses
Class D address range
224.0.0.0 to 224.0.0.255
224.0.1.0 to 231.255.255.255
233.0.0.0 to 238.255.255.255
232.0.0.0 to 232.255.255.255
Description
Reserved multicast addresses (IP addresses for
permanent multicast groups). The IP address 224.0.0.0
is reserved. Other IP addresses can be used by routing
protocols.
Available any-source multicast (ASM) multicast
addresses (IP addresses for temporary groups). They
are valid for the entire network.
Available source-specific multicast (SSM) multicast
group addresses.
1-6
Table 1-2.