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access-list

I saw the following access-list: 'access-list 3 permit 10.1.1.0 0.0.15.255' in a text book. How was the .15 in the wild card mask derived?

3 ACCEPTED SOLUTIONS

Accepted Solutions

Re: access-list

The actual subnet mask was : 255.255.240.0

The invert of this is : 0.0.15.255

To derive wildcard mask subtract the subnet mask's each octet from 255.

1.255-255 = 0

2.255-255 = 0

3.255-240 = 15

4.255-0 = 255

HTH..rate if helpful..

Re: access-list

Hello!

You can obtain the wildcard mask by replacing the 1 with 0 and 0 with 1 in the netmask binary.

Check the example below.

I know that the 0.0.15.255 in netmask is 255.255.240.0

So, let focus on the 3rd octet: 240

240 in binary is 11110000

we replace the 1 with 0 and 0 with 1 and we get 00001111 which is 15.

After a while with practice you will "see" automatically this netmask / wildcard stuff.

I hope you understand.

Cheers,

Calin

4 REPLIES

Re: access-list

The actual subnet mask was : 255.255.240.0

The invert of this is : 0.0.15.255

To derive wildcard mask subtract the subnet mask's each octet from 255.

1.255-255 = 0

2.255-255 = 0

3.255-240 = 15

4.255-0 = 255

HTH..rate if helpful..

New Member

Re: access-list

Thanks you.

Re: access-list

Hello!

You can obtain the wildcard mask by replacing the 1 with 0 and 0 with 1 in the netmask binary.

Check the example below.

I know that the 0.0.15.255 in netmask is 255.255.240.0

So, let focus on the 3rd octet: 240

240 in binary is 11110000

we replace the 1 with 0 and 0 with 1 and we get 00001111 which is 15.

After a while with practice you will "see" automatically this netmask / wildcard stuff.

I hope you understand.

Cheers,

Calin

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