The problem CIDR solves

Every device on an IP network needs an address. In IPv4, that address is 32 bits, usually written as four numbers from 0 to 255 separated by dots, like 192.168.1.10. The hard question is never "what is one address" but "how do I describe a range of addresses that belong together", a single office, a subnet, a customer allocation. CIDR notation is the compact way to write that range.

CIDR stands for Classless Inter-Domain Routing. The "classless" part matters: older addressing forced ranges into fixed sizes (the old Class A, B, and C blocks). CIDR removed that rigidity and let a range be any power-of-two size, which is why it is the standard today.

Reading the slash

A CIDR block looks like 192.168.1.0/24. The part after the slash, the prefix length, is the single most important number. It says: the first N bits of the address are fixed, and the rest are free to vary.

  • /24 means the first 24 bits are the fixed network part. The remaining 8 bits (32 − 24) identify individual hosts.
  • 8 free bits give 2⁸ = 256 possible combinations, so 192.168.1.0/24 describes the 256 addresses from 192.168.1.0 to 192.168.1.255.

Change the prefix and the block size changes with it. A smaller prefix means more free bits and a larger block:

  • /16 fixes 16 bits, leaves 16 free → 65,536 addresses.
  • /25 fixes 25 bits, leaves 7 free → 128 addresses.
  • /30 fixes 30 bits, leaves 2 free → 4 addresses.

Each step of one in the prefix length halves or doubles the block. That single rule is the whole intuition.

Network, broadcast, and usable hosts

Within most blocks, two addresses are special and not assignable to a host:

  • The network address is the first address in the block, all host bits set to 0 (192.168.1.0 for a /24). It names the block itself.
  • The broadcast address is the last address, all host bits set to 1 (192.168.1.255). Traffic sent here reaches every host in the block.

So a /24 has 256 total addresses but 254 usable for actual hosts. The "usable hosts" figure is simply total minus those two reserved addresses.

Two edge cases break this pattern, both deliberately:

  • A /31 (RFC 3021) is used for point-to-point links between two routers. It has only 2 addresses and, by design, both are usable, there is no room for a separate network and broadcast address, and none is needed for a link with exactly two ends.
  • A /32 is a single address, a host route. It describes exactly one machine.

Why this is worth understanding

Prefix length governs how networks are designed, how routes are summarized, and how address space is handed out. When you see 10.0.0.0/8 in a routing table or 0.0.0.0/0 as a default route, you are reading the same notation: a base address plus a prefix length that says how much of it is fixed. The arithmetic never changes, only the numbers.

You can try any block in the CIDR calculator to see its network address, broadcast address, mask, and usable host count computed instantly, entirely in your browser.