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copied!<p>You don't do it with your application.</p> <p><code>IGMP</code> is an internet layer protocol, it may not even reach your application.</p> <p>Whenever a unit wants to receive multicast to a certain address, it sends an <code>IGMP</code> request to join a group. A router receives the request and remembers that this user wants to belong to this group.</p> <p>Whenever the router receives a multicast packed destined for that address, it routes it to all the group members, possibly taking some access control restrictions into account.</p> <p>All group manupulation is performed by routers. You just send your <code>UDP</code> packets to a multicast address (that is <code>224/4</code>), and the routers decide whether to route it to a subscriber.</p> <p>If you want to limit destinations where your multicast packets go, you do it on routers.</p> <p>You should understand though, that the word <strong>"routes"</strong> above means that the router emits the packet into appripriate interface with a multicast destination address in <code>Ethernet</code> header and multicast destination address in <code>IP</code> header. An Ethernet switch attached to the interface, if any, will distribute the packet over <em>all</em> active ports. Since it knows nothing about internet routing, it will just see the broadcast/multicast bit set in the <code>Ethernet</code> header and act accordingly. </p> <p>There are, though, some <code>link layer</code> devices (Ethernet switches) who peek into <code>network layer</code> headers and limit multicast to the subscribed units. That is called <code>IGMP snooping</code>. Some of them can also be capable to control access.</p>

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