IPv4 multicast addresses are mapped to Layer 2 MAC addresses by a specific algorithm. The first 24 bits of the MAC address are fixed to 01:00:5E, and the subsequent 23 bits are derived from the lower 23 bits of the IPv4 multicast address. Since an IPv4 multicast address has 28 variable bits (after the fixed 1110 prefix), the upper 5 bits are ignored in this mapping. This creates a 32-to-1 ambiguity.

The addresses 239.99.12.35 and 229.227.12.35 map to the same MAC address because their lower 23 bits are identical. This is calculated by taking the last two octets (.12.35) and the last 7 bits of the second octet. For 227 (11100011), the last 7 bits are 1100011, which is 99. Thus, both map to 01:00:5E:63:0C:23. Using them simultaneously in the same L2 domain would cause traffic for both groups to be sent to subscribers of either group.

B. 237.87.12.35: The lower 23 bits of this address do not conflict with any other option, resulting in a unique MAC address among the choices.

C. 231.57.12.35: This address maps to a unique MAC address within this list as its lower 23 bits are distinct from the others.

E. 228.139.12.35: The lower 23 bits of this address are unique compared to the other options, so it does not create a MAC address collision.

1. Nokia 7450 ESS, 7750 SR, 7950 XRS, and VSR Multicast Routing Protocols Guide, Release 21.10.R1. In the "PIM" chapter, section "IP to MAC Address Translation", the guide states: "The last 23 bits are the last 23 bits of the IP multicast address. Because the first four bits of a multicast IP address are fixed and the next five bits are not used, there are 32 IP multicast addresses that can be mapped to a same MAC address." This directly supports the explanation of the mapping ambiguity.

2. IETF RFC 1112, "Host Extensions for IP Multicasting". Section 6.4, "Mapping of IP Multicast Addresses to Ethernet/FDDI Addresses", specifies the mapping algorithm: "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." This is the foundational standard defining the behavior.