Multicast MAC Addresses

What is Multicast MAC Address?

As you know, a MAC address consist of 48 bits. And these 48 bits are divided into two as vendor code and host code. The first part, vendor code is given by the device vendor. It shows the manufacturer of the device. The second part, host code identifies the device within the other devices of that vendor. There are also some specific MAC addresses used in IP Multicast Technology. These are Multicast MAC Addresses. So, what is a Multicast MAC address?

 

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Lets explain this usage with an example.

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For our example, we have a Multicast IP Address 224.255.0.1. Let’s find the Multicast MAC of this ip address and the other Multicast IP Addresses which use the same MAC address. Because, there are 32 Multicast IP addresses that corresponds to a Multicast MAC Address.

Here, we have 4 steps to produce this MAC address. These steps are given below:

 

1)Firstly, let’s write our ip address in binary format:

224.255.0.1 =  11100000.11111111.00000000.00000001  (32 bits)

 

3) We will take the fixed part of the Multicast MACs like below and we will write it in binary format. It is 25 bits.

0100:5e = 00000001:00000000:010111110:0

 

4) Lastly, we will combine the fixed 25 bits with the last 23 bits of the multicast ip to create our Multicat MAC Address :

00000001:00000000:01011110:01111111.00000000.00000001 = 01:00:5e:7F:00:01

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For Multicast IP Address, the first 4 bits are fixed, these first four bits are 1110. The last 23 bits are used for  Multicast MAC Address. And the remainning 5 bits in the middle can be any value. So, 2^5, you can create 32 different IP address with this changable area.

 

For our example, the Multicast IP addresses that share the same Multicast MAC Address are showed below. Here, with the 5th, 6th, 7th, 8th and 9th bits, 32 different Multicast IP Addresses can be written.

11100000.10000000.00110000.00001000

These Multicast IP Addresses that can be used with this created MAC Address are given below:

224.127.0.1
224.255.0.1
225.127.0.1
225.255.0.1
226.127.0.1
226.255.0.1
227.127.0.1
227.255.0.1
228.127.0.1
228.255.0.1
229.127.0.1
229.255.0.1
230.127.0.1
230.255.0.1
231.127.0.1
231.255.0.1
232.127.0.1
232.255.0.1
233.127.0.1
233.255.0.1
234.127.0.1
234.255.0.1
235.127.0.1
235.255.0.1
236.127.0.1
236.255.0.1
237.127.0.1
237.255.0.1
238.127.0.1
238.255.0.1
239.127.0.1
239.255.0.1