06-27-2008 07:04 PM - edited 03-03-2019 10:31 PM
Is there an easier way to understand the wildcard mask when configuring ACLs? I hardly understand how to get the correct wcm.
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06-27-2008 07:31 PM
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
06-27-2008 07:31 PM
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
06-27-2008 08:25 PM
Thanks that helps a lot, man this last book is a killer!
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