Why aggregate prefixes
Long address lists in firewalls and routers are slower and hard to maintain. Aggregation collapses adjacent and nested networks: 10.0.0.0/24 and 10.0.1.0/24 become 10.0.0.0/23, and duplicates and overlaps disappear. No address is added or lost.
Range to CIDR
An arbitrary range is usually not one subnet: 192.168.1.5–192.168.1.20 is 192.168.1.5/32, .6/31, .8/29, .16/30 and .20/32. We find the smallest set of such blocks.
Networks of a provider or a country
All prefixes of a provider are on the autonomous systems page; paste them here for a short firewall list. To split a network, use the subnet calculator.