200-301 Question #877: Real Exam Question with Answer & Explanation

The correct answer is A: /29. The subnet mask 11111111.11111111.11111111.11111000 has 29 bits set to '1', which corresponds to a /29 CIDR notation.

Submitted by olafpl· Mar 5, 2026

Question

A network engineer must configure an interface with IP address 10.10.10.145 and a subnet mask equivalent to 11111111.11111111.11111111.11111000. Which subnet mask must the engineer use?

Options

A/29
B/30
C/27
D/28

Explanation

The subnet mask 11111111.11111111.11111111.11111000 has 29 bits set to '1', which corresponds to a /29 CIDR notation.

Common mistakes.

B. A /30 mask represents 30 network bits (11111111.11111111.11111111.11111100), which is incorrect for the given binary mask.
C. A /27 mask represents 27 network bits (11111111.11111111.11111111.11100000), which is incorrect for the given binary mask.
D. A /28 mask represents 28 network bits (11111111.11111111.11111111.11110000), which is incorrect for the given binary mask.

Concept tested. Subnet mask CIDR notation conversion

Reference. https://www.cisco.com/c/en/us/support/docs/ip/routing-information-protocol-rip/13788-3.html

Topics

#Subnetting#CIDR notation#Binary subnet mask

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