What is CIDR notation and how does it work?

CIDR writes the network size as /x after the address, where x is the number of network bits — letting the split fall anywhere, not just on octet boundaries.

CIDR (Classless Inter-Domain Routing) replaced the rigid old "classes" (A=/8, B=/16, C=/24). The problem with classes was waste: an organisation needing 400 addresses had to take a whole Class B (65k). CIDR lets the network/host boundary sit at any bit position, so you allocate just the size you need.

Format: a.b.c.d/x, where x = how many leading bits are the network part; the remaining 32 − x bits are for hosts.

Example: 132.187.16.0/23

CIDR-ized Address:
10000100.10111011.0001000|0.00000000
└────── Network part ─────┘└─ Host ─┘
         (23 bits)          (9 bits)

Key concepts:

• /x specifies how many bits belong to the network
• Remaining bits (32 - x) are for hosts
• Replaces old classful addressing (Class A, B, C)

Common CIDR prefixes:

Go deeper:

• Classless Inter-Domain Routing (Wikipedia) — /x prefix length, the network/host boundary at any bit, and the subnet-mask relationship.