Greg Kirkness wrote: > > When converting an IP multicast address to a MAC multicast > address, why is > the bit immediately after the 01:00:5e always set to zero > instead of using > the highest-order bit from the IP address? > > Cheers >
There wouldn't be any reason to use the highest-order bit from the IP address, which of course would always be 1 since it's a Class D address. I think what you meant to ask is why didn't they use the high-order bit of the second octet? In other words why didn't they use 24 instead of 23 bits from the IP address? It would have reduced the number of IP multicast addresses that map to the same MAC address (which the IETF doesn't consider a serious problem anyway, though). According to Beau Williamson in his excellent book "Developing IP Multicast Networks," the reason was economics. Vendor codes (technically called Organizationally Unique Identifiers or OUIs) which are the first three octets of a MAC address used to cost $1000. So the developer of IP Multicast, Steve Deering, and his boss Jon Postel, could only afford one OUI, the 01:00:5E that we all know and love. But to make matters worse, Jon said that Steve could only use 1/2 of these addresses for his research into IP multicast. Jon paid the bills, I guess. Jon wanted to reserve the others for other research. He couldn't afford another OUI, but to make all his researchers happy, he only gave 1/2 of the addresses to Steve. That's the story anyway per Beau! It's that old 8th and 9th layer again (economics and office politics). Priscilla Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=51795&t=51787 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
