Tuesday, July 20, 2010

BSCI - Multicast - Work out IP Multicast MAC addresses

You can work out what the multicast MAC address is for a given multicast IP by using the following steps.

1) First stage is to remember that all multicast MAC addresses start 01-00-5e totalling 25bits (IANA allocated 24 bits plus an extra bit set to zero so this totals 25 bits). What remains is 23 bits to use for the MAC (48bits in total).

2) Take your multicast IP and right it out in binary e.g.
- = 1110 0000. 0 | 000 1010.0011 1100.0000 0011
                                                 23-bits to convert to a MAC

3) Now convert each group of 4 bits to Hex working from right to left
1110 0000. 0 | 000 1010.0011 1100.0000 0011
-                     0     a     3      c     0      3

4) Combine the converted Hex with the fixed multicast MAC bits:
- 01-00-5e-0a-3c-03

There you go. Relatively easy. Just remember that due to the 24 bits plus the zero bits that its best to work from right to left and you shouldn't go wrong.

I don't know, since I've not taken the exam yet, but to me this has exam question written all over it. Practice this so you can create multicast MAC addresses from a given multicast IP and you should have a mark in the bag.

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