6

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

  • 1
    This does not pass the "homework" sniff test. (and it's answered by the 30 year old RFC that defines the mapping.) – Ricky Beam Feb 19 '16 at 21:03
  • 1
    Basically, someone registered that OUI with the IEEE. Anyone can get an OUI. – Ron Maupin Feb 19 '16 at 21:43
  • Did any answer help you? if so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer. – Ron Maupin Aug 7 '17 at 19:19
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

| improve this answer | |
  • Basically, every multicast MAC address is used for 32 different IP multicast addresses (28 bits - 23 bits = 5 bits, and 2^5=32). – Ron Maupin Feb 19 '16 at 16:57
2

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.

| improve this answer | |

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.