RE: Question for Brian McGahan and others posted 12/21/2005
IMHO, while mathematically the XOR works perfectly fine it's generally a way
to make things appear much more complicated than they really are (or at
least have to be).
0 in mask means the bit must stay the same.
1 in mask means you don't care the value (e.g. there are some things you're
trying to match where it's a 1 and some where it's a 0)
Looking your columns there, the only entries you have variance are in the
3rd column (32-bit position) and the 6th colum (4-bit position). So if you
stick 1's in your mask (for "don't care") then you'll have a mask of 36.
An additional check you can do is to look at the number of "1" values in
your mask and take 2^x (where x = # of 1's). That will tell you how many
matches your mask will make. So in this case, the 1 in 32-bit and 1 in
4-bit positions makes two "1" bits in the mask. 2^2 = 4. There are 4
things you're trying to match anyway, so it's all cool.
Remember, no more, no less. If you find a fancy mask that let's in the
stuff you're looking for plus 16,000 of its closest friends why not save
yourself the headache and do "permit ip any any"? :)
PS. I'm into conserving brain cells... Blew too many of them in college!
From: nobody@xxxxxxxxxxxxxx [mailto:nobody@xxxxxxxxxxxxxx] On Behalf Of
Sent: Wednesday, December 21, 2005 8:57 PM
To: bmcgahan@xxxxxxxxxxxxxxxxxxxxxx; ccielab@xxxxxxxxxxxxxx
Subject: Question for Brian McGahan and others
I have the question regarding your paper on Computing Access-List and
There you have
I cannot get this trying in any way (for third column)
1) (((0 XOR 0) XOR 1) XOR 1)= ((0 XOR 1) XOR 1)= 1 XOR 1 =0
2) (0 XOR 0) XOR (1 XOR 1) = 0 XOR 0 = 0
I am doing something wrong or there is some other trick here ?
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