How do you recognize a multicast frame in Ethernet?

A multicast frame in Ethernet can be recognized by its MAC address. Specifically, the first octet of a multicast MAC address is significant; it must have the least significant bit (LSB) of the first octet set to 1. This means that the first octet will typically be an odd number, which identifies it as a multicast address.

For instance, common multicast addresses start from 01:00:00:00:00:00 to 01:7F:FF:FF:FF:FF, aligning with the characteristic of odd numbers in the first octet. Thus, the correct identification of a multicast frame is based on this setting in the MAC address.

The other choices do not describe the correct characteristics of multicast addresses. For example, an even last octet does not relate to the multicast property, while setting the first octet to zero is not indicative of multicast addresses, as that would typically denote a broadcast. Specific MAC addresses are more general and can refer to unicast addresses as well, lacking the specific identification feature required for multicast recognition.