Cisco Certified Network Associate (CCNA) Practice Exam

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What is the CIDR notation for a subnet mask of 255.255.255.224?

  1. /23

  2. /25

  3. /27

  4. /28

The correct answer is: /27

CIDR notation is used to denote the number of bits that are set to '1' in a subnet mask. The subnet mask of 255.255.255.224 can be broken down into binary: - The first three octets, 255.255.255, correspond to 11111111.11111111.11111111, which is 24 bits set to '1'. - The last octet, 224, is represented in binary as 11100000, which adds another 3 bits set to '1'. When you combine these, you have a total of 24 + 3 = 27 bits set to '1'. Therefore, the CIDR notation for the subnet mask of 255.255.255.224 is /27. The other options reflect incorrect sums of the bits in the subnet mask, as /23, /25, and /28 correspond to different configurations of subnet masks that do not match the binary representation of 255.255.255.224.

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