Elmer, Since an IP address needs to be mapped to a MAC address for delivery, a multicast frame needs a destination MAC address in the header. As a multicast frame is going to multiple destinations that are probably not known to the sender, a special MAC address needs to be used. After all, you can't fit multiple destination MAC addresses into an IP header. The base MAC address used for multicast is 0100.5E00.0000. In order to tailor the MAC address to the specific multicast group, the least significant 23 bits of the group address are mapped to the least significant 23 bits of the multicast MAC address.
The first four bits of the IP address (1110) identify it as a class D address (multicast) and will never change. Therefore, they are not considered when figuring the IP to MAC conversion. This leaves 28 bits. Since you are copying only 23 bits to the MAC address, you are left with 5 bits in the address that don't get used in thed MAC. Multicast IP address: 1110xxxx.xmmmmmmm.mmmmmmmm.mmmmmmmm x = un-used bits m = bits copied to the MAC address. Multicast MAC address: 0000000100000000.0101111000mmmmmm.mmmmmmmmmmmmmmmm m = bits copied from IP address That help? Karen *********** REPLY SEPARATOR *********** On 12/13/2001 at 12:00 Pm Elmer Deloso wrote: >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] Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=29117&t=29057 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
