How is Multicast IP mapped to Multicast Mac address?

Question

My main concern is why is the address 01-00-5E- and how? Especially 5 and E??

Answer 1

The 01-00-5E is commonly called the OUI (Organizationally Unique Identifier), which the IEEE (the group which assigns them) calls MA-L (MAC Address Block Large). Don't read a lot more into it than that. A MAC address does have two flag bits (the two least significant bits in the OUI), and the OUI for multicast correctly uses these bits to identify it as a universal, group MAC address.

The OUI for multicast was acquired from the IEEE by the creators of multicast. It cost $1000 for an OUI, which was a lot of money at the time, so the cost was split in half. Each of the two people who put up the money got to own half the addresses possible with the OUI. That means that of the 24 address bits (48 MAC address bits minus the 24 OUI bits), each person got 23 bits for addresses.

That's why the IP to MAC mapping only uses 23 bits when the multicast group part of an IP address requires 28 bits (Class D, multicast, addresses start with 1110 as the first four bits, leaving 28 of the 32 bits in an IP address for the multicast group). Because of mapping 28 bits into 23 bits, each multicast MAC address represents 32 different multicast IPv4 addresses.

Answer 2

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 (hex). Because there are 28 significant bits in an IP host group address, more than one host group address may map to the same Ethernet multicast address.

Source: RFC1112