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?
Multicast MACs starts with a fised part as 0100:5e. For one MAC address, only the last 23 bits of that MAC address is used to produce this Multicast MAC. The first 25 is fixed . The fixed part is 0100:5e (00000001:00000000:010111110:0 … ).
In this lesson, we will explain what is a Multicast MAC Address with an example. We will learn how to create these MACs and which Multicast IP addresses are using this same Media Access Control Address.
You can visit also Multicast Quizes page, to test yourself on Multicast Lessons.
Lets explain this usage with an example.
For our example, we have a Multicast IP Address 18.104.22.168. 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:
22.214.171.124 = 11100000.11111111.00000000.00000001 (32 bits)
2) Then, we let’s get the last 23 bits of this binary ip address :
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
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.
For example like 126.96.36.199 multicast ip address, 188.8.131.52, 184.108.40.206, 220.127.116.11 ip addresses will also use the same MAC that we have produced. This is tru for all these 32 ip addresses. For another Multicast MAC, there are another 32 ip addresses certainly.
In this lesson, we have learned an important lesson of multicast technology. We have talked about mac addresses used in multicast world and the 32 same ip addresses that is used with the same mac addresses.