Which two statements about subnet masks are true? (Choose two.)
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A. B. C. D.BC
Example: 255.0.0.0 (Class A network), in binary 11111111.00000000. 00000000. 00000000
The leftmost octet, 11111111, represents the network prefix, while the other three octets, with zeroes, represent the host portion.
Subnet masks are used to determine the network portion and host portion of an IP address. A subnet mask is a 32-bit value, just like an IP address, which is used to divide an IP address into two parts: the network prefix and the host address.
The network prefix identifies the network, and the host address identifies the specific host on that network. A subnet mask consists of a string of 1s followed by a string of 0s. The 1s indicate the network prefix, and the 0s indicate the host address.
The two statements that are true about subnet masks are:
A. If a bit is turned on (1), the corresponding bit in the IP address is used for the host portion. This statement is true because a 1 in the subnet mask indicates that the corresponding bit in the IP address is part of the network prefix, and therefore, the corresponding bit in the IP address is not used for the host portion. The bits in the IP address that are not part of the network prefix are used for the host portion.
C. If a bit is turned on (1), the corresponding bit in the IP address is used for the network prefix. This statement is also true because a 1 in the subnet mask indicates that the corresponding bit in the IP address is part of the network prefix, and therefore, the corresponding bit in the IP address is used for the network prefix. The bits in the IP address that are not part of the network prefix are used for the host portion.
On the other hand, option B and D are incorrect because they incorrectly describe the use of subnet mask bits. Specifically:
B. If a bit is turned off (0), the corresponding bit in the IP address is used for the host portion. This statement is false because a 0 in the subnet mask indicates that the corresponding bit in the IP address is part of the host portion, and therefore, the corresponding bit in the IP address is not used for the network prefix. The bits in the IP address that are not part of the host portion are used for the network prefix.
D. If a bit is turned off (0), the corresponding bit in the IP address is used for the network prefix. This statement is also false because a 0 in the subnet mask indicates that the corresponding bit in the IP address is part of the host portion, and therefore, the corresponding bit in the IP address is not used for the network prefix. The bits in the IP address that are not part of the host portion are used for the network prefix.