The first four bits of a multicast IP address are fixed as 1110 and are mapped to the most significant 25 bits of a multicast MAC address. Among the last 28 bits of a multicast IP address, only 23 bits are mapped to a multicast MAC address, and information about the other 5 bits is lost. As a result, 32 multicast IP addresses are mapped to the same MAC address.

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The first four bits of a multicast IP address are fixed as 1110, and the last 28 bits are used for mapping. Since only 23 of these 28 bits are mapped to the MAC address, 5 bits are lost, resulting in 2^5 = 32 different IP addresses mapping to the same MAC address.

The First Four Bits of a Multicast IP Address Are