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Subject Author Date
decrease MAC jammer 03-17-2008
Posted by jammer on March 17, 2008, 4:00 pm
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 I have a hex number which is actually a MAC address and I want to find
the MAC immediately before.
0000AAAA9999

Is there a way to treat it as a hex number and do -1 one on it?

Posted by Ben Morrow on March 17, 2008, 4:06 pm
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> I have a hex number which is actually a MAC address and I want to find
> the MAC immediately before.
> 0000AAAA9999
>
> Is there a way to treat it as a hex number and do -1 one on it?

my $MAC = '0000AAAA9999';
printf "%x\n", hex($MAC) - 1;

Ben


Posted by jammer on March 17, 2008, 4:37 pm
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>
> > I have a hex number which is actually a MAC address and I want to find
> > the MAC immediately before.
> > 0000AAAA9999
>
> > Is there a way to treat it as a hex number and do -1 one on it?
>
> my $MAC = '0000AAAA9999';
> printf "%x\n", hex($MAC) - 1;
>
> Ben

What if there are not 4 leading 0s.
my $MAC = '000BAAAA9999';
printf "%x\n", hex($MAC) - 1;
Integer overflow in hexadecimal number

Posted by Tad J McClellan on March 17, 2008, 7:38 pm
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>>
>> > I have a hex number which is actually a MAC address and I want to find
>> > the MAC immediately before.
>> > 0000AAAA9999
>>
>> > Is there a way to treat it as a hex number and do -1 one on it?
>>
>> my $MAC = '0000AAAA9999';
>> printf "%x\n", hex($MAC) - 1;
>>
>> Ben
>
> What if there are not 4 leading 0s.


You have completely missed the cause of the effect.


> my $MAC = '000BAAAA9999';


What _is_ significant is the number of bits required to represent the number.

ie. 32 bits required for Ben's number, but 36 bits are required
for your number.


> printf "%x\n", hex($MAC) - 1;
> Integer overflow in hexadecimal number


What do you know?

The message is exactly right, assuming you are on a 32-bit processor. :-)


--
Tad McClellan
email: perl -le "print scalar reverse qq/moc.noitatibaher0cmdat/"

Posted by Ben Morrow on March 17, 2008, 7:36 pm
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>
> > my $MAC = '000BAAAA9999';
>
> What _is_ significant is the number of bits required to represent the number.
>
> ie. 32 bits required for Ben's number, but 36 bits are required
> for your number.
>
> > printf "%x\n", hex($MAC) - 1;
> > Integer overflow in hexadecimal number
>
> What do you know?
>
> The message is exactly right, assuming you are on a 32-bit processor. :-)

And there are two solutions: use a perl built for 64-bit integers
(practically every processor nowadays has a 64-bit integral type, and
perl can usually use these if asked to); or use 'bigint', which will
allow arbitrary-sized integers at the expense of some speed.

Ben


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