Can multicast addresses collide at the MAC (Ethernet) layer ?
Yes. Normally, an Ethernet device will only accept packets addressed to its unique MAC identity (or to the broadcast address). However, Ethernet devices are also engineered to accept packets from the multicast MAC address range. MAC addresses are 48 bits; of that, 25 bits are used in the multicast prefix : multicast MAC addresses must start with 01:00:5E in hexadecimal, and the next bit must be zero. This leaves only 23 bits to store the multicast Class D address. Since multicast group addresses must start with 1110, there are really only 28 independent bits in a Class D address, and so 5 bits are “lost” in the translation to a MAC address, and collisions can occur. For a GLOP address in 233/8, a total of 8 bits of the Class D address are redundant, and so only one bit is lost (the highest order bit of the AS number, see below). Since no autonomous system has yet been assigned a ASN with that bit set to one, GLOP addresses will not yet be subject to MAC collisions (at least with other
