PLANET WGSW-50040 - Manual (Page 156)

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in Multicast IP address.
Since only 23bits out of the lower 28bits in IP Multicast address are mapped into MAC address, therefore
there are 32 IP Multicast addresses which are mapped into the same MAC address.
21.1.3 IP Multicast Packet Transmission
In Multicast mode, the source host sends packets to the host group indicated by the Multicast group address
in the destination address field of IP data packet. Unlike Unicast mode, Multicast data packet must be
forwarded to a number of external interfaces to be sent to all receiver sites in Multicast mode, thus Multicast
transmission procedure is more complicated than Unicast transmission procedure.
In order to guarantee that all Multicast packets get to the router via the shortest path, the receipt interface of
the Multicast packet must be checked in some certain way based on Unicast router table; this checking
mechanism is the basis for most Multicast Routing Protocol to forward in Multicast mode --- RPF (Reverse
Path Forwarding) check. Multicast router makes use of the impressed packet source address to query Unicast
Router Table or independent Multicast Router Table to determine if the packet ingress interface is on the
shortest path from receipt site to source address. If shortest path Tree is used, then the source address is the
address of source host which sends Multicast Data Packets; if Shared Tree is used, then the source address
is the address of the root of the Shared-Tree. When Multicast data packet gets to the router, if RPF check
passes, then the data packet is forwarded according to Multicast forward item, and the data packet will be
discarded else wise.
21.1.4 IP Multicast Application
IP Multicast technology has effectively solved the problem of sending in single point and receiving in
multipoint. It has achieved the effective data transmission from a point to multiple points, saved a great deal of
network bandwidth and reduced network load. Making use of the Multicast property of network, some new
value-added operations can be supplied conveniently. In Information Service areas such as online living
broadcast, network TV, remote education, remote medicine, real time video/audio meeting, the following
applications may be supplied:
1) Application of Multimedia and Streaming Media
2) Data repository, finance application (stock) etc
3) Any data distribution application of “one point to multiple points”
In the situation of more and more multimedia operations in IP network, Multicast has tremendous market
potential and Multicast operation will be generalized and popularized.