CIDR (Classless Inter-Domain Routing)
CIDR (RFC 4632) replaced the old class-based (A/B/C) system for allocating IP address blocks with a flexible prefix-length notation, written as an address followed by a slash and a bit count (for example 192.168.1.0/24), where the prefix marks how many leading bits identify the network. This lets a block be sized to the number of hosts actually needed instead of only the fixed class sizes, and lets routers summarize many smaller blocks into one advertised route, which is what kept the global routing table from growing unmanageably as the internet scaled. The same slash notation is also used for subnetting within a single organization's own network, not just for internet-wide routing. A /24 block provides 256 addresses and a /32 is a single host; the lower the prefix number, the larger the block it describes.
By AdminKit team, Netbase JSC · Updated