How IP Addresses Work
What Is CIDR Notation?
That little slash-number after an IP address — like /24 or /16 — is CIDR notation, the modern way to describe a block of addresses.
The problem CIDR solved
The original IPv4 system used fixed "classes." A Class A network gave you about 16 million addresses; a Class C gave you 256. There was almost nothing in between. An organization that needed a few thousand addresses had to take a wastefully huge Class A or juggle many small Class C blocks. This squandered the limited IPv4 space.
Classless addressing
Classless Inter-Domain Routing (CIDR) threw out the rigid classes. Instead, a block is described by an address plus a prefix length: 203.0.113.0/24. The number after the slash says how many leading bits are fixed as the network portion. Everything else identifies hosts.
How to read the slash
/24fixes 24 bits, leaving 8 for hosts → 256 addresses./16fixes 16 bits, leaving 16 for hosts → 65,536 addresses./8fixes 8 bits, leaving 24 for hosts → about 16.7 million addresses.
The rule of thumb: a smaller number after the slash means a bigger block. Each step down doubles the size.
Why it matters
CIDR lets address space be handed out in right-sized pieces, which conserves the scarce IPv4 supply. It also enables route aggregation: instead of advertising thousands of tiny networks, providers can advertise one summarized block, keeping the internet's routing tables smaller and faster.
CIDR in everyday life
You will meet CIDR notation in router settings, firewall rules ("allow 192.168.0.0/16"), cloud configurations, and the public "whois" records that show which organization owns a block of addresses. Once the slash clicks, a lot of networking documentation stops looking like a foreign language.