From: sabrina pittarel (sabri_esame@yahoo.com)
Date: Thu Nov 30 2006 - 05:09:35 ART
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
This archive was generated by hypermail 2.1.4 : Fri Dec 01 2006 - 08:05:49 ART