Subject: Re: [Magma] question

From: Marshall Eubanks <[email-masked]>
Date: Wed, 5 Sep 2001 07:21:30 -0400

>hi,
>
>the mac addr is hard-coded on to the card ... how then can one map the 28 

>bits of the ip addr onto 23 bits of the mac addr ?

Hello Hitesh;

   I hope things are well. The Class D to Ethernet MAC mapping
is described in 
http://www.multicasttech.com/faq/multicast_faq.html
amoung many other places. Collisions are indeed possible.

>
>also bit 0 in octet 7 has to be set to 1 for broadcast/multicast ... how is

>this bit set ?
Bit zero of Octet zero of any MAC address is zero for Layer 2
unicast, one for layer two multicast. (Every OUI thus contains
lots of multicast addresses!) The 23 bit for multicast is
for the mapping of Internet group addresses to MAC, and
is a historical accident.

Note that Ethernet MACs are big-endian, and so bit 0 of 
octet 0 is actually the 8th bit encountered.

Marshall

>rgds
>hitesh
Marshall Eubanks

[email-masked]