Ah, I see.
They wanted to give him a half from the 24 bits, right? Which isn’t 12 because we’re making the calculation in bits where each bit effectively doubles the MAC address count. So by removing one bit, we shrink the size by half.
David
Hello David
Yes, that’s exactly it! That’s the nature of binary numbers. By removing the most significant bit (i.e. the leftmost bit) you essentially cut the address space in half…
Laz
I’m so sorry guys, but can one told me how I need to map these addresses ?
If I want to use multicast for my network I just choose a random ip address 239.x.x.x. ?
where the mac address comes from?
I thinks I got the maths, but not really sure how to start
Hello Daniel
When you enable multicast in a topology using multicast routers, you typically configure such features at Layer 3, that is the multicast IP level.
The issues involved with multicast MAC addresses are not typically configured directly. These multicast MACs are automatically calculated and used internally, so you really don’t have to configure them. The lesson here is more informational so you know how they are calculated, so if you need to do troubleshooting or you see such MAC addresses in the MAC address table, you’ll know what they are.
Concerning configuring multicast groups, yes, you typically choose a random multicast address as long as it is within the acceptable IPv4 multicast address space.
I hope this has been helpful!
Laz
Hello, everyone.
This one was rather tougher to understand at first, but I think I get it now. When we’re calculating the range of IPs that a single MC MAC address corresponds to, we basically tweak the 5 blue bits
Since a multicast MAC is 23-bits long, it won’t be big enough to account for the 5 bits in the 28-bit IP address. This means that one MAC will correspond to 32 different IPs. Since you have 5 bits to tweak, and if the remaining 23-bits are stable, the MAC address effectively remains the same because it doesn’t provide enough bits.
Also writing it down in ranges is rather complex and should be avoided. If I’ve said that the following IPs map to the same MAC:
224.0.0.5 - 239.128.0.5
You could assume that even 239.127.0.5 would map to the same MAC as I’ve specified it as a range 
I suppose it always makes more sense to either write it all down or remember the fact that only 32 IPs map to one multicast MAC which means 16 IPs if the second octet is 0 and another 16 IPs if the second octet is 1, or just write it like this:
224-239.0.0.5
224-239.128.0.5
Thanks!
David
Hello David
Indeed, the explanation you gave in your post is spot on.
At first I didn’t understand what you meant, but now I see that you’re referring to Rene’s “faster method” of determining the addresses that correspond to a single multicast MAC address. Indeed referring to them as a range can be a bit misleading. I’ll ask Rene to take a look at that part of the lesson to see if he can clarify it.
Laz