CIDR
acronymnetworking
Stands for: Classless Inter-Domain Routing
A compact way to write an IP network as an address plus a prefix length, like 10.0.0.0/8.
Classless Inter-Domain Routing (RFC 4632) replaced the old rigid A/B/C classes with a slash number saying how many leading bits are the network. It is the notation behind almost every subnet decision, and the input our CIDR tools compute against.
CIDR replaced a system where address blocks came in three fixed sizes, and organizations that needed slightly more than one size received the next one up, wasting the difference. Allowing the boundary between network and host to fall anywhere ended that waste and, more importantly, made route aggregation possible: many adjacent networks can be advertised as one prefix, which is the only reason the global routing table remains a manageable size.
In daily work CIDR is mostly arithmetic done under time pressure. Whether a host falls inside a range, how many addresses a prefix actually provides after the network and broadcast addresses are subtracted, and whether two blocks overlap are the questions that firewall rules, ACLs, and cloud security groups turn on. They are also easy to get subtly wrong at three in the morning, which is exactly why a deterministic calculator beats mental arithmetic.
The notation carries a mental model worth internalizing. A shorter prefix means fewer network bits, more host bits, and a larger block; each additional bit halves the size. Once that inverse relationship is automatic, reading a routing table stops requiring conscious translation.
Also known as: cidr, slash notation, prefix length