Binary Math - Part I Answ ...
    17 September 08

    Binary Math - Part I Answers

    Posted byINE

    Thank you to everyone who participated...  It was my first time running a little contest on the blog, and I'm sorry to say it didn't quite work as I expected!  The comments were not supposed to be seen until a day later, but I think I forgot to share that with the other folks here!  My bad!

    Anyway, there are a variety of answers that we received in the commentary, and remember that I said all three must be correct.  That was the catchy part, as even the first few people were almost there.  Almost, but not quite!  It's a good learning curve though!

    Andrew Dempsey was the first person to actually get all three of them correct!  Congratulations!  Andrew, pop me an e-mail and we'll figure out how to get the tokens to you.

    To everyone else, I promise to have all the kinks worked out by tomorrow when I post Part II with a much more exciting set of things to think about!


    1.  Start picking a few and finding similarities again...

    0    00000000
    4    00000100
    8    00001000
    12    00001100
    16    00010000
    20    00010100

    Just like with our even/odd example, we look at the constants.  The last two bits will ALWAYS be "00".  So a mask of 11111100 would fit.

    access-list 16 permit

    2.  Our differences here are in the third octet.

    128    10000000
    130    10000010
    132    10000100
    134    10000110
    136    10001000
    138    10001010

    There are three bits of difference between the six values there.  The 2-bit, 4-bit and 8-bit positions.  But 2^3 would yield eight matches.

    140 and 142 are missing there.  So we have two ways of looking at this.

    Method 1:

    access-list 17 permit
    access-list 17 permit

    Method 2:

    access-list 17 deny
    access-list 17 permit

    Either way, two lines is our best bet!

    3.  This is a little more complicated than it looks!  Namely because there's a gap in the middle and we aren't allowed to use any "deny" statements!  Go figure!

    This exercise is just like creating subnets though.  How well did you know your bit boundaries?  :)

    So let's make as many major blocks as we can.

    access-list 18 permit
    access-list 18 permit
    access-list 18 permit
    access-list 18 permit
    access-list 18 permit
    access-list 18 permit
    access-list 18 permit
    access-list 18 permit
    access-list 18 permit

    After a while you have to think about all the different bit boundaries.  On the bright side, Windows Calculator available to help if necessary!

    Back in a day'ish!

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