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What Is CIDR Notation? Subnets, IP Ranges, and Network Masks Explained

Understand CIDR notation from scratch — how /24, /16, and other prefix lengths work, how to calculate IP ranges, and how to use CIDR for network configuration and cloud security groups.

What Is CIDR Notation?

CIDR (Classless Inter-Domain Routing) is a compact way to represent an IP address and its network mask: 192.168.1.0/24, 10.0.0.0/8, 0.0.0.0/0.

The number after the slash is the prefix length — how many bits are fixed (network part). The remaining bits are variable (host addresses).

How It Works

192.168.1.0/24

IP in binary:   11000000.10101000.00000001.00000000
                |_____ 24 bits fixed _____|__ 8 bits variable __

/24 means:

  • First 24 bits are the network: 192.168.1
  • Last 8 bits vary: 0 to 255
  • Total addresses: 2^8 = 256 (254 usable — .0 is network address, .255 is broadcast)

Quick formula:

  • Total hosts = 2^(32 - prefix_length)
  • Usable hosts = total - 2

Common CIDR Blocks

CIDR Usable IPs Common use
/32 1 Single IP (allowlist one host)
/30 2 Point-to-point link
/28 14 Very small subnet
/27 30 Small team subnet
/26 62 Small office
/25 126 Half class C
/24 254 Standard network (home, small office)
/22 1,022 Medium network
/20 4,094 Large subnet
/16 65,534 Large organization
/8 16.7 million Huge network
/0 All Entire internet ("allow all" in firewall)

Private IP Ranges (RFC 1918)

Not routable on the public internet:

CIDR Range Typical use
10.0.0.0/8 10.x.x.x Large corporate
172.16.0.0/12 172.16.x.x – 172.31.x.x Medium networks
192.168.0.0/16 192.168.x.x Home/office

CIDR in Cloud Security Groups

# AWS Security Group inbound rules:
Allow SSH (22)        from 203.0.113.0/24   → only your office can SSH
Allow HTTPS (443)     from 0.0.0.0/0        → anyone on internet
Allow PostgreSQL (5432) from 10.0.0.0/16    → only VPC instances

VPC Subnet Design Example

VPC: 10.0.0.0/16  (65,536 addresses)

Public subnets:
  10.0.1.0/24  (us-east-1a) — 254 hosts — load balancers
  10.0.2.0/24  (us-east-1b) — 254 hosts

Private subnets:
  10.0.10.0/24 (us-east-1a) — 254 hosts — app servers
  10.0.11.0/24 (us-east-1b) — 254 hosts

Database subnets:
  10.0.20.0/24 (us-east-1a) — 254 hosts — RDS
  10.0.21.0/24 (us-east-1b) — 254 hosts

Calculating IP Range Manually

For 192.168.5.0/26:

  • Hosts = 2^(32-26) = 2^6 = 64
  • Usable = 62
  • Network address = 192.168.5.0
  • Broadcast = 192.168.5.63
  • Usable range = 192.168.5.1 to 192.168.5.62

→ Calculate subnet ranges automatically with the IPv4 Subnet Calculator.