Confusion on wildcard masks

Answered Question
Aug 30th, 2007

What exactly is the wildcard mask doing in this line:

access-list 10 deny 192.168.10.128 0.0.0.31

Thanks

I have this problem too.
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Correct Answer by Mohamed Sobair about 9 years 4 months ago

Hi Guys,

For the Wildcard mask, I have a best practise to calculate it as follows:

1- you take the original Subnetmask of the Network ID and Subtract it by(255.255.255.255).

Example:

you have the Network : 192.168.10.128/27

192.168.10.128 255.255.255.224

** always take (255.255.255.255) -

(255.255.255.224)

equals = 0.0.0.31 (wildcard mask)

So the wildcard mask for 192.16.10.128/27

equals 192.168.10.128 0.0.0.31, which will match 31 bit hosts from 129 - 159.

Note: the last octet 159 is the broadcast of the subnet & its included in the calculation.

Regards,

Mohamed Sobair

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purohit_810 Thu, 08/30/2007 - 07:00

It will deny all IP address start from

192.168.10.128 to 139 {129 and 140 both will comver}

Block size is 0 to 31 mean it covers 32 Ip address.

Regards,

Dharmesh Purohit

Correct Answer
Mohamed Sobair Thu, 08/30/2007 - 10:46

Hi Guys,

For the Wildcard mask, I have a best practise to calculate it as follows:

1- you take the original Subnetmask of the Network ID and Subtract it by(255.255.255.255).

Example:

you have the Network : 192.168.10.128/27

192.168.10.128 255.255.255.224

** always take (255.255.255.255) -

(255.255.255.224)

equals = 0.0.0.31 (wildcard mask)

So the wildcard mask for 192.16.10.128/27

equals 192.168.10.128 0.0.0.31, which will match 31 bit hosts from 129 - 159.

Note: the last octet 159 is the broadcast of the subnet & its included in the calculation.

Regards,

Mohamed Sobair

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