300-320 Question #555: Real Exam Question with Answer & Explanation

The correct answer is D: 32. One multicast MAC address can represent 32 different multicast IP groups. IPv4 multicast addresses (224.0.0.0–239.255.255.255) use 28 unique bits, but the IEEE multicast MAC address format (01:00:5E:xx:xx:xx) maps only the lower 23 bits of the IP multicast address into the MAC. T

Question

How many multicast groups can one multicast MAC address represent?

Options

A1
B128
C16
D32

Explanation

One multicast MAC address can represent 32 different multicast IP groups. IPv4 multicast addresses (224.0.0.0–239.255.255.255) use 28 unique bits, but the IEEE multicast MAC address format (01:00:5E:xx:xx:xx) maps only the lower 23 bits of the IP multicast address into the MAC. This creates a 5-bit ambiguity (28 − 23 = 5), meaning 2^5 = 32 different IPv4 multicast group addresses can hash to the same Layer 2 multicast MAC address. This is why switches use IGMP snooping rather than relying solely on MAC addresses to forward multicast traffic accurately.

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