Mask Conversion Problem 5: Answer
The answer is /21.
For problem 5, mask 255.255.248.0 converts to the following:
11111111 11111111 11111000 00000000
You can see from the binary number that it contains 21 binary 1s, so the prefix format of the mask will be /21.
If you memorized the number of binary 1s represented by each DDN mask value, you can possibly work faster with the following logic:
Step 1. Start with a prefix value of 0.
Step 2. (1st octet) Add 8 because the first mask octet of 255 includes eight binary 1s.
Step 3. (2nd octet) Add 8 because the second mask octet of 255 includes eight binary 1s.
Step 4. (3rd octet) Add 5 because the third mask octet of 248 includes five binary 1s.
Step 5. The resulting prefix is /21.
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