CCNA Data Center DCICN 640-911 Official Cert Guide

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.