Networking - Security - Discussion

Discussion :: Security - Security (Q.No.10)

10. 

You want to create a standard access list that denies the subnet of the following host: 172.16.144.17/21. Which of the following would you start your list with?

[A]. access-list 10 deny 172.16.48.0 255.255.240.0
[B]. access-list 10 deny 172.16.144.0 0.0.7.255
[C]. access-list 10 deny 172.16.64.0 0.0.31.255
[D]. access-list 10 deny 172.16.136.0 0.0.15.255

Answer: Option B

Explanation:

First, you must know that a /21 is 255.255.248.0, which is a block size of 8 in the third octet. Counting by eight, this makes our subnet 144 in the third octet, and the wildcard for the third octet would be 7 since the wildcard is always one less than the block size.

Rizwan said: (Aug 4, 2015)  
How we get 144 in third octet here?

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