From: Ryan (ryan95842@gmail.com)
Date: Thu Nov 30 2006 - 17:38:47 ART
(use courier to view)
This is the same information I've read in a what seems like a dozen books
and web pages. They all say that you only use last 23bits of the IP to make
up the MAC address. Nothing new here. Yet, there is is this 32:1 overlap,
but I've not found any documentation or book that goes beyond that to
explain.
So if I try this out for myself for example:
Lets take a random Multicast IP address. 228.45.3.2
(I just typed in some numbers, so that's as random as were going to get)
Let's make this IP into a Multicast MAC address...
228.45.3.2
01:00:5e:xx:xx:xx
xxx.45.3.2
45 . 3 . 2
0010 1101.0000 0011.0000 0010
^
2 D: 0 3: 0 2
x010 1101.0000 0011.0000 0010
So if I've done my math right, 228.45.3.2 should be: 01:00:5e:2d:03:02
If I add a '128' to it to simulate the 1 in the 24th spot that gets excluded
in the conversion,
228.173.3.2 should ALSO be: 01:00:5e:2d:03:02
So, based on this, I would have to say that for any given Multicast IP
address, there is going to be EXACTLY 32 duplicate MAC address's.
224.y.x.x
224.(y+128).x.x
225.y.x.x
225.(y+128).x.x
...
238.y.x.x
238.(y+128).x.x
239.y.x.x
239.(y+128).x.x
So, what happens with well know Multicast IP's like the 224.0.0.x address's?
This would seem to mean that you could not use any Multicast address is
y.0.0.x range or y.128.0.x range as it would conflict with the
224.0.0.xaddress's.
This correct?
-Ryan
On 11/29/06, sabrina pittarel <sabri_esame@yahoo.com> wrote:
>
>
>
> Hi Rayn,
> the way the MAC Destination Address for a multicast packet is built is
the following:
>
> the last 23 bits of the IP multicast address are copied into the last 23
bits of the MAC address, while the beginning of the MAC address is fixes and
set to 0100.5e(xx.xxxx)
> This leaves 9 bits of the IP multicast address (32-23 = 9) that are not
reflected anyhow in the L2 MAC. These are the first 9 bits of the IP
address.
> Of these 9 bits 4 are fixed (1110b => class D address) but 5 are variable
so all the combinations of these 5 bits (2^5) generates ip multicast
addresses that matches the same mac.
>
> Hope this helps,
>
> Sabrina
>
>
>
>
>
> ----- Original Message ----
> From: Ryan <ryan95842@gmail.com>
> To: ccielab@groupstudy.com
> Sent: Wednesday, November 29, 2006 6:02:17 PM
> Subject: Grasping the Multicast 32:1 duplicate MAC address issue...
>
>
>
> I'm a little confused by the Multicast duplicate MAC address issue...
>
> Is the 32:1 duplicate MAC address issue just for certain address's (32
> total)?
>
> 224.1.1.1
> 224.129.1.1
> 225.1.1.1
> 225.129.1.1
> ...
> 238.1.1.1
> 238.129.1.1
> 239.1.1.1
> 239.129.1.1
>
> Or does it apply to ANY Multicast address? So for any one Multicast
address,
> there are exactly 32 duplicates....
>
>
>
> -Ryan
>
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