Q1: 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?

ANS:B - access-list 10 deny 172.16.144.0 0.0.7.255

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.