CCENT/CCNA ICND1 100-101 Official Cert Guide

Mask Conversion Problem 4: Answer

The answer is /15.

For problem 4, mask 255.254.0.0 converts to the following:

11111111 11111110 00000000 00000000

You can see from the binary number that it contains 15 binary 1s, so the prefix format of the mask will be /15.

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 7 because the second mask octet of 254 includes seven binary 1s.

Step 4. The resulting prefix is /15.