Multicast MAC address

Multicast MAC address

This may or may not be particularly useful, but I found it interesting, so I decided to blog about it. Hopefully, this will help someone have a better understanding of multicast and how packets flow over the network. All packets on an IP network need to have a MAC address so the network can send the frames between layer 2 devices. This makes sense, but I was a bit surprised to find out the multicast sessions have their own MAC address. How else would multicast traffic and packets get delivered? I guess I never took the time to even think about it before.

I have always thought and talked about the multicast session in the form of an IP address like 230.230.0.1 but each MC session has a MAC address associated with it. The session would need this to move the traffic on a layer 2 network.

This blog will explain how the MAC address is created; I will show you some screenshots displaying these multicast MAC addresses. This might be confusing at first but once you read it over a few times it makes perfect sense.

All Multicast MAC addresses start 01:00:52:0 (the 25th bit in the MAC address for a Multicast session is always 0) the next 23 bits are the IP address translated to binary or Hex. The table below shows the MAC address for the multicast IP session of 230.230.0.1

230 230 0 1
IP address in Binary form 11100110 11100110 00000000 00000001
MAC address in Binary form 00000001 00000000 01011110 01100110 00000000 00000001
MAC Address 01 00 5E 66 00 01

There is an issue when you convert multicast IP into a MAC address. When we look at this a bit deeper you will see that since the 25th bit is always 0 this MAC address for 230.230.0.1 and 230.102.0.1 would be the same. Both MAC addresses would be 01:00:5e:66:00:01 I have never seen this cause an issue before but theoretically, it could.

230 102 0 1
IP address in Binary form 11100110 01100110 00000000 00000001
MAC address in Binary form 00000001 00000000 01011110 01100110 00000000 00000001
MAC Address 01 00 5E 66 00 01

Packet capture of a multicast MAC address

I work for Vocera so naturally, the packets I have captured are Vocera Multicast sessions. The Vocera Multicast group MAC address is normally 230.230.xxx.xxx The first session will have 230.230.0.1 and each session will increase by one. The range we use has 230.230.0.1 – 230.230.15.254 when the server runs out of addresses it will start over. Stopping and starting the server or a fail-over will reset the address’s space as well.

Vocera MC MAC address is always 01:00:5e:66:xx:xx xx:xx matches up with the 3rd and 4th Octet in the IP address. As the graph above shows 230.230.0.1 MAC address is 01:00:5e:66:00:01 the last Multicast session 230.230.15.254 would be 01:00:5e:66:0f:fe

I hope you found this interesting and thought-provoking. Thank you for reading this blog. Please leave comments and continue this discussion on Twitter and Slack. If you haven’t followed me on Twitter please use this link to follow me.

 

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