Richard, This is an excellent post, but i need a little bit of clarification on... 1. I've understood multicast as at Layer 3, so I'm confused when you say that a "25-bit prefix is assigned" for the Layer 2 frame. I can't seem to follow what is happening in multicast addressing between the Layers 2-3 to arrive at this 25-bit prefix. I can't figure out where to place this prefix bit setting while looking at the 802.3 frame format on Uyless Black's book on Data Link Protocols. 2. You state "there is a short fall of five bits and 2 to the 5 is 32". What is this 2^5 referring to? 3. Finally, "are all allocated a MAC of 0100.5e01.0101." Please confirm... is this the Destination MAC on the DA field of the frame? If so, what happens when you have to pass this multicast stream of data from one interface to another, e.g. from mBone -> r1 -> r2 -> multicast enabled Intranet endstations, will the same "multicast MAC address" stay the same?
Thanks for your input. Elmer Deloso -----Original Message----- From: richard beddow [mailto:[EMAIL PROTECTED]] Sent: Thursday, December 13, 2001 9:18 AM To: [EMAIL PROTECTED] Subject: RE: about multicast address! [7:29057] An IP m'cast address is 32 bits long (as with any IP address), the first for bits are 0x1110 leaving 28 bits. (Still with me :)) Any m'cast ethernet borne frame has a 48 bit MAC (as do all ethernet frames). A 25 bit prefix is assigned leaving 23 bits. As 28 won't go into 23 there must be some duplication, there is a short fall of five bits and 2 to the 5 is 32. Hence and one m'cast MAC represents 32 IP addresses. For instance 224.1.1.1 224.128.1.1 225.1.1.1 225.128.1.1 etc etc 238.1.1.1 238.128.1.1 239.1.1.1 239.128.1.1 are all allocated a MAC of 0100.5e01.0101. Hope this is explained OK. RB Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=29095&t=29057 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
