Which two statements about mapping multicast IP addresses to MAC addresses are true? (Choose two.)

Question

Options

• AAll mapped multicast MAC addresses begin with 0x0100.5E
• BThe router performs the mapping before it hands the packet off to a switch
• CAll multicast MAC addresses end with 0x0100.5E
• DThe mapping process may generate overlapping addresses, which can cause receivers to receive
• EAll destination MAC addresses begin with an octet of binary 1s

• A
  94% (30)
• C
  3% (1)
• E
  3% (1)

Explanation

IPv4 multicast addresses (224.0.0.0/4) are mapped to Ethernet MAC addresses using a well-defined process. (A) is true: all multicast MAC addresses begin with 01:00:5E (hex), which is the IANA-assigned OUI for IPv4 multicast. The 25th bit of the MAC is always 0, giving a 23-bit space for the address mapping. (D) is true: the mapping only uses the low-order 23 bits of the 32-bit multicast IP address. Since multicast IP addresses have 28 variable bits (the first 4 bits are always 1110), 5 bits of the IP address are lost in the mapping. This means 32 different multicast IP group addresses map to the same Ethernet MAC address, potentially causing hosts to receive multicast traffic for groups they did not join. Choice C is false (they BEGIN with 01:00:5E, they do not end with it). Choice E describes the broadcast MAC (FF:FF:FF:FF:FF:FF), not multicast.

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