CCENT/CCNA ICND1 100-101 Official Cert Guide

Problem 4

Step 1. The problem statement tells us to use /26.

Step 2. The subnets would be 172.16.0.0, 172.16.0.64, 172.16.0.128, 172.16.0.192, 172.16.1.0, 172.16.1.64, and so on, counting by 64 in the fourth octet and by 1 in the third octet.

Step 4. All the beginning /26 subnets overlap with the large first existing subnet (172.16.0.0/20). Although you could write down all the /26 subnets, you would need to write down 64 of these subnet numbers before getting to 172.16.16.0, which is the first number past the range for the first existing subnet.

Step 5. 172.16.16.0/26 is the numerically lowest new subnet number that does not overlap with the existing subnets.