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Answered Question
Jun 27th, 2008
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Is there an easier way to understand the wildcard mask when configuring ACLs? I hardly understand how to get the correct wcm.



Correct Answer by Jon Marshall about 8 years 11 months ago

Chris


Easiest way for me is to flip the bits from the normal subnet mask (sb) eg.



sb -> 255.255.255.0 = wcm -> 0.0.0.255

sb -> 255.255.0.0 = wcm -> 0.0.255.255


They are the easy ones, where there is a 255 in the sb make it a 0 for the wcm and where there is a 0 in the sb make it a 255 in the wcm.


To come up a wildcard mask for others eg.


sb -> 255.255.255.128 = wcm = 0.0.0.127

sb -> 255.255.255.192 = wcm = 0.0.0.63

sb -> 255.255.255.248 = wcm 0.0.0.7


quick way to work out - take the octet that isn't 255 and subtract it from 256 then subtract 1 ie.


sb -> 255.255.255.192 =


256 - 192 = 64 - 1 = 63 so


wcm = 0.0.0.63


sb -> 255.255.224.0 =


256 - 224 = 32 - 1 = 31 so


wcm = 0.0.31.255


Hope that makes sense


Jon



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Correct Answer
Jon Marshall Fri, 06/27/2008 - 19:31
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Chris


Easiest way for me is to flip the bits from the normal subnet mask (sb) eg.



sb -> 255.255.255.0 = wcm -> 0.0.0.255

sb -> 255.255.0.0 = wcm -> 0.0.255.255


They are the easy ones, where there is a 255 in the sb make it a 0 for the wcm and where there is a 0 in the sb make it a 255 in the wcm.


To come up a wildcard mask for others eg.


sb -> 255.255.255.128 = wcm = 0.0.0.127

sb -> 255.255.255.192 = wcm = 0.0.0.63

sb -> 255.255.255.248 = wcm 0.0.0.7


quick way to work out - take the octet that isn't 255 and subtract it from 256 then subtract 1 ie.


sb -> 255.255.255.192 =


256 - 192 = 64 - 1 = 63 so


wcm = 0.0.0.63


sb -> 255.255.224.0 =


256 - 224 = 32 - 1 = 31 so


wcm = 0.0.31.255


Hope that makes sense


Jon



chrisblaze Fri, 06/27/2008 - 20:25
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Thanks that helps a lot, man this last book is a killer!

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