You've seen IP addresses written like 192.168.1.0/24 or 10.0.0.0/8. That slash number is CIDR notation — a compact way to describe a range of addresses. Once you can read it, networking documentation suddenly makes sense.

What CIDR is

CIDR (Classless Inter-Domain Routing) replaced the old class A/B/C system in 1993. It lets you carve up IP space into ranges of any size by specifying how many bits of the address are the network prefix. The rest are host addresses inside that network.

How to read /N

The number after the slash is how many of the leading bits are the network ID. The remaining bits identify hosts.

• /32 → exactly 1 IP address
• /24 → 256 addresses (8 host bits, 2⁸ = 256)
• /16 → 65,536 addresses
• /8 → 16,777,216 addresses
• /0 → all 4.3 billion IPv4 addresses

Each step down in the prefix doubles the size of the range.

Common CIDRs you'll see

• 10.0.0.0/8 — full private Class A range
• 172.16.0.0/12 — full private Class B range
• 192.168.0.0/16 — full private Class C range
• 192.168.1.0/24 — typical home network
• 0.0.0.0/0 — "any IP" (default route)

How firewalls and routers use CIDR

Allow rules, deny rules, route tables, VPN configurations — all use CIDR to express "this range of IPs" in one line. allow from 203.0.113.0/24 means "all 256 addresses starting at 203.0.113.0."

Quick mental math

Prefix length plus host bits always equals 32 (IPv4) or 128 (IPv6). To find the host count: 2^(32 − prefix). Subtract 2 for usable hosts (network and broadcast addresses are reserved). For /24: 2⁸ − 2 = 254 usable.

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