I could have explained this above but then you would have skipped through this part which does make a difference. Last 7 bits of 64 which still equals 64 converted to binary = 1000000Īfter removing 1 bit from our binary result it still equals 64 so this is what we will be using in the next step. Since we are only using the last 7 bits from the second octet, just drop the first 0 or 1 from our binary number (first binary number on the left, the result from 128 above). So above we converted the second octet of our multicast IP which was 64 to decimal which equal 01000000. You keep doing this until you can no longer subtract from the octet and you have converted decimal to binary. Can 64 be subtracted from 64? Yes, write a 1. Can 128 be subtracted from 64? No, write a 0. A quick way to do this is by following this example below. In the example I will be using the second octet of the multicast IP 239.64.100.1 which would be 64. The first step is to convert the second octet of the multicast IP to binary. You should have a good understanding of IP sub-netting or this may get a bit confusing.Ĭonvert Second Octet of Multicast IP to Binary To determine the rest of the MAC address we will use the remaining 23 low order bits to generate the full MAC address. The high order 25 bits of the MAC address are fixed and that is the 01-00-5E. As I explained above, any MAC address you see that starts with 01-00-5E is related to a multicast address. There is a specific range assigned for multicast, 01-00-5E-00-00-00 to 01-00-5E-7F-FF-FF for Ethernet and Fiber Distributed Data Interface (FDDI). I’ll explain below exactly how to determine the multicast MAC address from the multicast IP as well as provide a few helpful websites.Īt the layer 2 level whenever multicast is used it has to be assigned a MAC address in order to work. I knew that any mac address that started with 01-00-5E was a multicast address, but wasn’t sure how the rest of it was calculated. If it happens accidentally, a packet from a different IP multicast application can be identified at Layer 3 and discarded however, network administrators should be careful when they implement multicast applications so that they can avoid using IP addresses that produce identical MAC addresses.Recently I was working to track down a multicast problem for a client on an enterprise network and needed to determine what the exact multicast MAC address would be. IETF points out that the chances of two multicast applications on the same LAN producing the same MAC address are very low. In fact, because 5 bits from the IP address are always mapped to 0, 25 (32) different class D IP addresses produce exactly the same MAC address. Unfortunately, this method does not provide a unique multicast MAC address for each multicast IP address, because only the last 23 bits of the IP address are mapped to the MAC address. Step 6 Combine the first 6 hexadecimal digits 01-00-5E with the last 6 hexadecimal digits, calculated in Step 5, to form a complete multicast MAC address of 12 hexadecimal digits. Step 5 Convert the last 24 bits of the multicast MAC address from binary to 6 hexadecimal digits. Step 4 Copy the last 23 bits of the binary IP address in the last 23-bit space of the multicast MAC address. Step 3 Replace the next 5 bits of the binary IP address with one binary 0 in the multicast MAC address space. Step 2 Replace the first 4 bits 1110 of the IP address with the 6 hexadecimal digits (or 24 bits) 01-00-5E as multicast OUI, in the total space of 12 hexadecimal digits (or 48 bits) for a multicast MAC address. Notice the first 4 bits they are always 1110 for any multicast IP address. To understand the mechanics of this process, use the following six steps, which are referenced by number in Figure 19-6: The method is identical for Ethernet and Fiber Distributed Data Interface (FDDI).įigure 19-6 Calculating a Multicast Destination MAC Address from a Multicast Destination IP AddressĠ 0 0 1 0 1 0. The MAC address is formed using an IEEE-registered OUI of 01005E, then a binary 0, and then the last 23 bits of the multicast IP address. Figure 19-6 shows how a multicast MAC address is calculated from a Layer 3 multicast address. Assigning a Layer 3 multicast address to a multicast group (application) automatically generates a Layer 2 multicast address.
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