3Com WX3000 operation manual IP multicast address, Class D address range Description

As receivers are multiple hosts in a multicast group, you should be concerned about the following questions:

zWhat destination should the information source send the information to in the multicast mode?

zHow 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:

zThe membership of a group is dynamic. A host can join and leave a multicast group at any time.

zA multicast group can be either permanent or temporary.

zA multicast group whose addresses are assigned by IANA is a permanent multicast group. It is also called reserved multicast group.

Note that:

zThe IP addresses of a permanent multicast group keep unchanged, while the members of the group can be changed.

zThere can be any number of, or even zero, members in a permanent multicast group.

zThose 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.

Table 1-2Range and description of Class D IP addresses

Class D address range	Description
	Reserved multicast addresses (IP addresses for permanent
224.0.0.0 to 224.0.0.255	multicast groups). The IP address 224.0.0.0 is reserved.
	Other IP addresses can be used by routing protocols.
224.0.1.0 to 231.255.255.255	Available any-source multicast (ASM) multicast addresses
	(IP addresses for temporary groups). They are valid for the
233.0.0.0 to 238.255.255.255
	entire network.
232.0.0.0 to 232.255.255.255	Available source-specific multicast (SSM) multicast group
	addresses.

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