networkingaddressing

CIDR

cidrnetworkingaddressing
Plain English

CIDR is the short way to say how big a network is. In 192.168.1.0/24, the /24 means the first 24 bits name the network and the rest name devices, so it holds 254 usable addresses.

Technical Definition

CIDR (RFC 4632) replaced fixed class A, B and C networks with a variable prefix length.

  • /24 = 255.255.255.0, 256 addresses, 254 hosts.
  • /16 = 255.255.0.0, 65,536 addresses.
  • /30 = 4 addresses, 2 hosts (point-to-point links).
  • /32 is one host; /0 is everything.

Hosts = 2 to the power of (32 minus prefix), minus 2 for network and broadcast. IPv6 uses the same notation, usually /64 per LAN.

Source: RFC 4632; CompTIA Network+ N10-009 Exam Objectives v4.0

CIDR in practice

$ ip -4 addr show
    inet 10.0.2.15/24 metric 100 brd 10.0.2.255 scope global dynamic enp0s2

$ sudo ufw allow from 192.168.1.0/24 to any port 22
In the Wild

You meet CIDR in every ip addr output, firewall rule, and router setting. Network+ N10-009 objective 1.7 expects you to subnet with it. In BNLC Phase 5 you read it from your own VM.

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