L3 multicast to L2 multicast conversion

Hi,
In Cisco BSCI Official Exam Certification Guide, in "Do I Know This Already" section, I didn't understand the answer to question 7.
Any ideas?

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ccnpninja

oops, I forgot to mention the page: it's 461.
thanks

nicklauscombs

ill check this out for you at lunch

nicklauscombs

*edited a few times, hopefully I'm not rambling here

*

just to make sure I'm looking at the right one, this question is "Which of the following multicast addresses do not share a multicast MAC address?"

if that's the one you were looking at here's the "short" answer - if you don't know the theory behind the conversion let me know and I'll try to explain better, if you do understand but just didn't understand why the answer was right hopefully this clears it up

looking at the second octet of each ip address, you don't use the first octet or the first bit of the second octet when converting (which is why i made the big space gap there) so 145, 17 and 17 all will convert to the same hex because of the similar bits, don't forget you have to look at the third and fourth octets as well but the challenge comes from comparing the second octet since you don't convert the first bit in it (too easy if the third or fourth octets are all different) so I would imagine any question asked on the exam will be set up exactly like this. since all of the bits used from the second, third and fourth octets are the same, the first three answers will have the same multicast MAC address. the fourth answer of 117 will have a different hex conversion because the bits are different from the first three answers thus making a different multicast MAC address, does that make sense? (whew that was long winded!)

145 - 1_____0010001

17 - 0_____ 0010001

17 - 0______0010001

117 - 0______1110101

ccnpninja

nicklauscombs, you rock! Now I see.

nicklauscombs

excellent! I'm glad that made sense