Problem 3
Step 1. The problem statement tells us to use /21.
Step 2. The subnets would be 172.16.0.0, 172.16.8.0, 172.16.16.0, 172.16.24.0, and so on, counting by 8 in the third octet.
Step 3. The first two new possible subnets (172.16.0.0/21, 172.16.8.0/21) overlap with the first existing subnet. 172.16.16.0/21 overlaps with three other existing subnets, and 172.16.32.0/21 overlaps with the last subnet listed in Table H-5.
Step 4. 172.16.24.0/21 is the numerically lowest new subnet number that does not overlap with the existing subnets.
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