IP multicast
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... group, that is permanent; at any time a
permanent group may have any number of members, even zero. Those IP
multicast addresses that are not reserved for permanent groups are
...
... be mapped to the well-known local address of an IP multicast router;
a router on such a network ...
... timer expires first.
Note that the multicast routers receive all IP multicast
datagrams, and therefore need not be addressed explicitly.
...