Compute the most efficient summary route (aggregate route) for a set of IP prefixes.
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Both terms describe combining multiple specific routes into a single shorter advertisement. CCNA material uses 'summarization' inside an AS; BGP material uses 'aggregation' between ASes.
A summary route holds steady even when one of its specific subnets flaps. The neighbours never witness the flap, so the rest of the network stays insulated from convergence cycles.
If the summary covers addresses that aren't actually reachable through this router, traffic meant for those addresses ends up on the wrong device—and gets dropped.
Convert the boundary addresses to binary and walk left-to-right until the bits diverge. The number of matching bits is the longest common prefix length.