The figure illustrates that summarizing networks into a single address and mask can be done in three steps:
Step 1. List the networks in binary format. In the example, the two area 1 networks 10.1.1.0/24 and 10.1.2.0/24 are listed in binary format.
Step 2. Count the number of far left matching bits to determine the mask for the summary route. As highlighted, the first 22 far left matching bits match. This results in the prefix /22 or subnet mask 255.255.252.0.
Step 3. Copy the matching bits and then add zero bits to determine the summarized network address. In this example, the matching bits with zeros at the end result in a network address of 10.1.0.0/22. This summary address summarizes four networks: 10.1.0.0/24, 10.1.1.0/24, 10.1.2.0/24, and 10.1.3.0/24.
In the example the summary address matches four networks although only two networks exist.