Some summarized routes combine many routes into one route, but that might not be the best summarization.  The best summarization means that the summary should include all the subnets specified in the question but as few other addresses as is possible.

The following list describes a generalized binary process by which you can find a best summary route for a group of subnets:

Step 1: List all to-be-summarized subnet numbers in binary.

Step 2: Find the first N bits of the subnet numbers for which every subnet has the same value, moving from left to right.

Step 3: To find the summary router’s subnet number, write down the in-common bits from Step 2 and binary 0s for the remaining bits.  Convert back to decimal, 8 bits at a time, when finished.

Step 4: To find the summary route’s subnet mask, write down N binary 1s, with N being the number of in-common bits found at Step 2.  Complete the subnet mask with all binary 0s.  Convert back to decimal, 8 bits at a time, when finished.

Step 5: Check your work by calculating the range of valid IP addresses implied by the new summary route, comparing the range to the summarized subnets.  The new summary should encompass all IP addresses in the summarized subnets.

By looking at the subnets in binary, you can easily discover the bits in common among all the subnet numbers.  By using the longest number of bits in common, you can find the best summary.