Step 1Convert the addresses to binary format and align them in a list.
Step 2Locate the bit where the common pattern of digits ends. (It might be helpful to draw a vertical line marking the last matching bit in the common pattern.)
Step 3Count the number of common bits. The summary route number is represented by the first IP address in the block, followed by a slash, followed by the number of common bits, the first 22 bits of the IP addresses from 172.16.12.0 through 172.16.15.255 are the same. Therefore, the best summary route is 172.16.12.0/22.