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IP/ARP Extensions for IP Multicasting

To support IP multicasting, an additional route is defined on the host. The route (added by default) specifies that if a datagram is being sent to a multicast host group, it should be sent to the IP address of the host group through the local interface card, and not forwarded to the default gateway. The following route (which you can discover using the route print command) illustrates this: Network Address Netmask Gateway Address Interface Metric 1 Host group addresses are easily identified, as they are from the class D range, to These IP addresses all have 1110 as their high-order bits. To send a packet to a host group, using the local interface, the IP address must be resolved to a media access control address. As stated in the RFCs: "An IP host group address is mapped to an Ethernet multicast address by placing the low-order 23 bits of the IP address into the low-order 23 bits of the Ethernet multicast address 01-00-5E-00-00-00 (hex). Because there are 28 significant bits in an IP host group address, more than one host group address may map to the same Ethernet multicast address." For example, a datagram addressed to the multicast address would be sent to the (Ethernet) media access control address 01-00-5E-00-00-05. This media access control address is formed by the junction of 01-00-5E and the 23 low-order bits of (00-00-05). Because more than one host group address can map to the same Ethernet multicast address, the interface may indicate hand-up multicasts for a host group for which no local applications have a registered interest. These extra multicasts are discarded by TCP/IP.

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