Route summarization, also known as route aggregation, is the process of advertising a contiguous set of addresses as a single address with a less-specific, shorter subnet mask. CIDR is a form of route summarization and is synonymous with the term supernetting.
CIDR ignores the limitation of classful boundaries, and allows summarization with masks that are smaller than that of the default classful mask. This type of summarization helps reduce the number of entries in routing updates and lowers the number of entries in local routing tables. It also helps reduce bandwidth utilization for routing updates and results in faster routing table lookups.
In the figure, R1 requires a summary static route to reach networks in the range of 172.20.0.0/16 to 172.23.0.0/16.