From: nenad pudar (nenad.pudar@gmail.com)
Date: Wed Dec 21 2005 - 23:17:05 GMT-3
This is the way I used to think too
Since the bit at certain position do not match we need to declare them
"not-care" and have 1 in wildcard mask.
I was a little bit confused by this this paper I found
nenad
On 12/21/05, Scott Morris <swm@emanon.com> wrote:
>
> 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"? :)
>
> HTH,
>
> Scott
>
> PS. I'm into conserving brain cells... Blew too many of them in college!
>
> -----Original Message-----
> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
> nenad pudar
> Sent: Wednesday, December 21, 2005 8:57 PM
> To: bmcgahan@internetworkexpert.com; ccielab@groupstudy.com
> Subject: Question for Brian McGahan and others
>
> Hi Brian
> I have the question regarding your paper on Computing Access-List and
> Wildcard Pairs
>
> There you have
>
> 00000000
> 00000100
> 00100000
> 00100100
> _________
> XOR
> 00100100=36****
>
> 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 ?
>
>
> thanks
> nenad
>
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