Chapter 1 Introduction
WX Series User’s Guide
20
Figure 14
Multicast Example
In the multicast example above, systems A and D comprise one multicast group. In multicasting, the
server only needs to send one data stream and this is delivered to systems A and D.
IGMP (Internet Group Multicast Protocol) is a network-layer protocol used to establish membership in a
Multicast group - it is not used to carry user data. IGMP version 2 (RFC 2236) is an improvement over
version 1 (RFC 1112) but IGMP version 1 is still in wide use. IGMP version 3 supports source filtering,
reporting or ignoring traffic from specific source address to a particular host on the network.
A class D IP address is used to identify host groups and can be in the range 224.0.0.0 to 239.255.255.255.
The address 224.0.0.0 is not assigned to any group and is used by IP multicast computers. The address
224.0.0.1 is used for query messages and is assigned to the permanent group of all IP hosts (including
gateways). All hosts must join the 224.0.0.1 group in order to participate in IGMP. The address 224.0.0.2 is
assigned to the multicast routers group.
At start up, the WX Device queries all directly connected networks to gather group membership. After
that, the WX Device periodically updates this information.
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