With a /19 prefix length, what is the corresponding subnet mask?

With a /19 prefix length, what is the corresponding subnet mask?

• A. 255.255.224.0
• B. 255.255.255.0
• C. 255.0.0.0
• D. 255.255.255.255

Answer: (A)

Understanding how a prefix length maps to a subnet mask helps you translate /19 into a dotted decimal mask. A /19 means the first 19 bits of the 32-bit address are the network portion. That fills the first two octets completely (8 bits each = 16 bits) and leaves 3 bits in the third octet as part of the network. Those three bits set to 1 give 11100000 in binary for the third octet, which is 224 in decimal. The fourth octet is all zeros. Put together, the subnet mask is 255.255.224.0. This mask corresponds to a /19. Other masks map to different prefixes: 255.255.255.0 is /24, 255.0.0.0 is /8, and 255.255.255.255 is /32 (a single host). So 255.255.224.0 is the correct match for a /19.

Understanding how a prefix length maps to a subnet mask helps you translate /19 into a dotted decimal mask. A /19 means the first 19 bits of the 32-bit address are the network portion. That fills the first two octets completely (8 bits each = 16 bits) and leaves 3 bits in the third octet as part of the network. Those three bits set to 1 give 11100000 in binary for the third octet, which is 224 in decimal. The fourth octet is all zeros. Put together, the subnet mask is 255.255.224.0.

This mask corresponds to a /19. Other masks map to different prefixes: 255.255.255.0 is /24, 255.0.0.0 is /8, and 255.255.255.255 is /32 (a single host). So 255.255.224.0 is the correct match for a /19.