To calculate a decimal subnet mask from a CIDR prefix length (e.g., $/24$), follow these steps: Step 1: Understand the prefix length. The prefix length (e.g., $N$ in $/N$) indicates the number of network bits from the left in a 32-bit IPv4 address. The remaining $32 - N$ bits are host bits. Step 2: Write the subnet mask in binary. Create a 32-bit binary number. The first $N$ bits will be $1$s, and the remaining $32 - N$ bits will be $0$s. Step 3: Divide the binary mask into four 8-bit octets. Group the 32 binary bits into four sets of 8 bits. Step 4: Convert each binary octet to its decimal equivalent. Convert each 8-bit binary number into its decimal value. These four decimal values, separated by dots, form the dotted-decimal subnet mask. Example: Calculate the decimal subnet mask for $/26$. Step 1: The prefix length is $N=26$. This means there are 26 network bits and $32 - 26 = 6$ host bits. Step 2: Write the 32-bit binary mask: $$ \underbrace{11111111}_{8 \text{ bits}} \underbrace{11111111}_{8 \text{ bits}} \underbrace{11111111}_{8 \text{ bits}} \underbrace{11000000}_{8 \text{ bits}} $$ (26 ones followed by 6 zeros) Step 3: Divide into four octets: • First octet: $11111111$ • Second octet: $11111111$ • Third octet: $11111111$ • Fourth octet: $11000000$ Step 4: Convert each octet to decimal: • $11111111_2 = 255_{10}$ • $11111111_2 = 255_{10}$ • $11111111_2 = 255_{10}$ • $11000000_2 = (1 \times 128) + (1 \times 64) + (0 \times 32) + (0 \times 16) + (0 \times 8) + (0 \times 4) + (0 \times 2) + (0 \times 1) = 128 + 64 = 192_{10}$ The decimal subnet mask for $/26$ is $\boxed{255.255.255.192}$. That's 2 down. 3 left today — send the next one.