🔢 CIDR Notation Explained: Why It Replaced Class A/B/C Networks

What CIDR Notation Actually Says

CIDR (Classless Inter-Domain Routing) notation writes an IP address followed by a slash and a number — like 192.168.1.0/24 — where the number after the slash states exactly how many of the address's 32 bits are considered part of the network portion. Everything after those leading network bits is the host portion, available for assigning individual devices within that subnet. A /24 means the first 24 bits (three full octets) are network, leaving 8 bits (one octet) for hosts; a /26 means 26 bits are network, leaving only 6 bits for hosts.

Why the Older Class System Existed First

Before CIDR, IPv4 addressing used a rigid class-based system: Class A networks used a fixed /8 prefix (16.7 million addresses per network), Class B used a fixed /16 prefix (65,536 addresses), and Class C used a fixed /24 prefix (256 addresses) — with the address's own leading bits determining which class, and therefore which fixed prefix length, it belonged to. This system was simple to implement in early routing hardware but had no flexibility: an organization needing 300 addresses had no option between a Class C (254 usable, too few) and a Class B (65,534 usable, wildly excessive) — there was no size in between.

Why This Rigidity Became a Real Problem

As the internet grew through the 1980s and early 1990s, the class system's lack of granularity caused two compounding problems: massive address-space waste (many organizations were allocated a full Class B network for a few hundred or few thousand actual hosts, since that was the only size available above Class C) and explosive growth in the global internet routing table (each allocated network, regardless of its actual size or how it related to other allocations, needed its own routing table entry, since there was no way to represent multiple adjacent allocations as a single summarized route).

What CIDR Actually Fixed

CIDR, introduced in the early 1990s (formalized in RFC 1518 and RFC 1519), eliminated the fixed class boundaries entirely, allowing a network prefix of any length from /0 to /32 — this let network operators allocate exactly the address space a given need actually required (a /27 for 30 hosts, a /22 for over a thousand, anything in between) rather than being forced into one of three fixed sizes. Just as importantly, CIDR enabled route summarization (or "supernetting") — combining multiple contiguous smaller allocations into a single larger routing table entry when they share a common prefix, directly addressing the routing-table growth problem that motivated CIDR's development in the first place.

Reading a CIDR Block at a Glance

Once the prefix-length concept is clear, reading a CIDR block tells you immediately how large that network is without needing to consult any external chart: subtract the prefix from 32 to get host bits, then 2^(host bits) gives total addresses in the block (2^(host bits) − 2 gives usable host addresses, since the network and broadcast addresses are reserved, as covered in the companion network/broadcast address article). A /30 (2 host bits) is a 4-address block with 2 usable hosts — the standard size for a point-to-point router link. A /22 (10 host bits) is a 1,024-address block with 1,022 usable hosts — a common size for a mid-sized office VLAN.

Why CIDR Notation Still Dominates Modern Network Documentation

Despite being decades old, CIDR notation remains the standard way network engineers document, configure, and discuss IP address ranges — router configurations, firewall rules, VLAN documentation, and IP address management (IPAM) systems all express subnet sizes in CIDR form rather than reverting to older dotted-decimal subnet mask notation alone, precisely because the compact /n format immediately conveys both the address range and its size in a single, unambiguous token.