Subnet masks are used in IP networking to define the boundary between the network portion and the host portion of an IP address. A valid subnet mask must consist of a contiguous block of 1s followed by a contiguous block of 0s in its binary representation.

Let’s analyze the given options:

A. 255.192.0.0– This isnot a standard or valid subnet maskbecause the 1s are not contiguous beyond the second octet. This is typically used in class A subnetting but is not commonly considered valid in CIDR or MTCNA context. While technically binary-valid, it’s not recommended or standard for practical subnetting.

B. 255.255.192.255–Invalid, because the last octet is255, which implies all bits are 1s, but in the third octet only partial bits are set (192is11000000). This breaks the required rule of contiguous 1s followed by contiguous 0s.

C. 192.0.0.0–Invalid, as it doesn’t represent a valid subnet mask.192in the first octet (11000000) followed by zeros is not a valid mask – it's actually a network address, not a subnet mask.

D. 255.255.224.0–Valid subnet mask. This represents/19in CIDR notation. In binary:11111111.11111111.11100000.00000000, which follows the correct rule of contiguous 1s followed by contiguous 0s.

Extract from MTCNA Study Guide by René Meneses:

Subnet masks must be a continuous string of 1s followed by a continuous string of 0s. Any deviation or split between the blocks renders the mask invalid.

Extract from MTCNA Official Course Manual:

Valid subnet masks include values such as 255.0.0.0 (/8), 255.255.0.0 (/16), 255.255.255.0 (/24), and also non-classful masks like 255.255.224.0 (/19) are allowed and used for more flexible subnetting.

Conclusion:Option D is the only one meeting the criteria for a valid subnet mask as taught in the MTCNA curriculum.

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