Wildcard mask question

Answered Question
Aug 8th, 2007
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Hi i have this wildcard mask that i cannot understand:

0.0.31.254 i cant understand the 254 part, does this mean that it converts to 255.255.224.1 mask? i never seen a mask like that before.

it is taken from this ACL:

access-list 5 deny 172.26.192.0 0.0.31.254


Correct Answer by sundar.palaniappan about 9 years 9 months ago

Actually that's a valid WC mask. That's the beauty of WC masks they are so flexible that you could almost match anything.


.254 indicates it doesn't care about the first 7 bits in the last octet but the last bit (1 bit) has to match always. With an address 172.26.192.(0) the last bit is turned off therefore any address with the last bit turned off i.e any even # 0,2,4,6 etc.. will match this WC mask.


HTH


Sundar

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Correct Answer
sundar.palaniappan Wed, 08/08/2007 - 07:19
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Actually that's a valid WC mask. That's the beauty of WC masks they are so flexible that you could almost match anything.


.254 indicates it doesn't care about the first 7 bits in the last octet but the last bit (1 bit) has to match always. With an address 172.26.192.(0) the last bit is turned off therefore any address with the last bit turned off i.e any even # 0,2,4,6 etc.. will match this WC mask.


HTH


Sundar

Pavel Bykov Wed, 08/08/2007 - 07:38
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You have to understand WC as not reversed subnet mask, but as bit sequence. If you convert the wild card to it's bit value.

Where there is 1, no match is required.

Where there is 0, a match is required.

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