Multicast MAC Addresses

Multicast-MAC-addres-creation

Multicast MAC Addresses

Multicast MAC addresses are the MAC addresses start with 0100:5e . As you know, MAC address has 48 bits and consist of vendor code and host code. But for one MAC address, only the last 23 bits of that MAC address is used to provide multicast MAC address. The first 25 is fixed . The fixed part is 0100:5e (00000001:00000000:010111110:0 … )

Lets explain Multicast MAC Addresses with an example.


Multicast-MAC-addres-creation
We have a Multicast IP Address 224.255.0.1 . Let’s find the Multicast MAC address of this address and the other Multicast IP Addresses which has the same MAC address. Because, there are 32 Multicast IP addresses that corresponds to a Multicast MAC Address.

224.255.0.1 =  11100000.11111111.00000000.00000001  (32 bits)Our Multicast Address’s fixed part will be like below.

0100:5e = 00000001:00000000:010111110:0We will add the last 23 bits of our Multicast IP address to that fixed part.

1111111.00000000.00000001Our Multicat MAC Address is below:

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


multicast-MAC-example
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 examples, the Multicast IP addresses that share the same Multicast MAC address is showed below. Here, with the 5th, 6th, 7th, 8th and 9th bits, 32 different Multicast IP Addresses can be written.

11100000.10000000.00110000.00001000


Multicast-MAC-combinations
These Multicast IP Addresses that can be used with this created Multicast 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

Lesson tags: Multicast MAC Addresses
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