What is the subnet mask for a /24 network in decimal notation?

Correct answer: 255.255.255.0

Option D is correct because a /24 prefix length means the first 24 bits are the network portion, leaving 8 bits for hosts. In dotted-decimal notation, 24 bits of ones followed by 8 bits of zeros yields 255.255.255.0. Option A (255.255.255.128) represents a /25 subnet, which uses 25 bits for the network portion, not 24. Option B (255.0.0.0) represents a /8 or Class A default mask, covering only 8 network bits. Option C (255.255.0.0) represents a /16 or Class B default mask, covering 16 network bits.