Prefix aggregation
IPv4 Subnetting Reference
CIDR provides the possibility of fine-grained routing prefix aggregation, also known as supernetting or route summarization. For example, sixteen contiguous /24 networks (formely Class C) can be aggregated and advertised to a larger network as a single /20 route, if the first 20 bits of their network addresses match. Two aligned contiguous /20s may then be aggregated to a /19, and so forth. This allows a significant reduction in the number of routes that have to be advertised.
IPv4 CIDR
(*)Note that for routed subnets bigger than /31 or /32, 2 needs to be subtracted from the number of available addresses – the largest address is (conventionally[citation needed]) used as the broadcast address, and typically the smallest address is used to identify the network itself. See RFC 1812 for more detail. It is also common for the gateway IP for that subnet to use an address, meaning that you would subtraction from the number of usable hosts that can be used on the subnet.
