Classless Inter-Domain Routing (CIDR)
The IP addressing and routing scheme that replaced fixed address classes with prefixes of any length, written as an address, a slash and the number of network bits.
Classless Inter-Domain Routing, specified in RFC 4632, ended the fixed class A, B and C boundaries of early IPv4. CIDR lets the boundary between network and host fall at any bit. In CIDR notation, such as 192.168.10.0/24, the figure after the slash counts the leading bits that belong to the network portion; here that is 24 bits, leaving 8 bits for hosts, the same split as the subnet mask 255.255.255.0.
It lets address space be allocated in sizes that fit real needs, which slowed IPv4 exhaustion, and lets routers aggregate contiguous networks into one summary route, which keeps routing tables smaller. It is the notation behind subnetting, and IPv6 uses prefix lengths in the same way. For security, CIDR is the language of firewall rules and access lists, so an error in one prefix changes how many hosts a rule covers. A /16 written where a /32 was meant opens a rule to 65,536 addresses rather than one, and 0.0.0.0/0 means any IPv4 address.
Exam relevance: questions in this area tend to give a prefix and ask how large the network is, or which rule is broader. Candidates are expected to read a prefix length as a count of network bits and to recognise that a smaller number after the slash means a larger network.