Problem 1

Step 1. The problem statement tells us to use /30.

Step 2. The subnets would be 172.16.0.0, 172.16.0.4, 172.16.0.8, and so on, counting by 4 in the fourth octet and counting by 1 in the third octet.

Step 4. Many possible /30 subnets overlap: all 64 that begin 172.16.0, all 64 that begin 172.16.1, and so on, through all the subnets that begin 172.16.15 because the first existing somewhat-large subnet has a range from 172.16.0.0 to 172.16.15.255.

Step 5. 172.16.16.0/30 is the numerically lowest new subnet number that does not overlap with the existing subnets.

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