Plain-language meaning
CIDR notation writes an IP network with an address and a prefix length, such as 192.168.1.0/24. The number after the slash tells how many leading bits belong to the network portion.
Example
An IPv4 /24 network has 2^(32 - 24), or 256, total addresses. In many ordinary subnetting contexts, some addresses may be reserved for network and broadcast use.
Why CIDR matters
CIDR replaced older class-based network boundaries and makes it possible to allocate address blocks more flexibly. Smaller prefix numbers usually mean larger address blocks; larger prefix numbers usually mean smaller blocks.
Limitations
Subnet calculators can show address ranges and counts, but network design also depends on routing, security rules, DHCP, cloud-provider reservations and IPv4 vs IPv6 behavior.
FAQ
Is /24 larger than /28?
Yes. In IPv4, /24 has more addresses than /28 because fewer bits are fixed for the network portion.
Does CIDR work for IPv6?
Yes, CIDR-style prefix notation is also used for IPv6, but the address length is 128 bits instead of 32.