CIDR

Definition

CIDR (Classless Inter-Domain Routing) is a notation used to define IP ranges and subnet sizes.

What Problem It Solves

CIDR solves the rapid depletion of IPv4 addresses under rigid, wasteful classful boundaries (Class A, B, C). It allows network administrators to allocate subnets of any size (flexible bit-masking), minimizing the waste of unused IP addresses.

What Happens If Not Used

Without CIDR, the global pool of IP addresses would have run out decades ago, and small networks would be forced to allocate unnecessarily large IP ranges, causing severe routing table bloat across the internet.

Easy Wording

CIDR tells how many IP addresses are in a network and which addresses belong to it.

Layman Example

Suppose hotel says:

Rooms

100 - 199

Everyone knows:

100 Rooms

Belong to same block

CIDR is similar.

Example:

192.168.1.0/24

means:

192.168.1.0
to
192.168.1.255

Technical Example

Firewall rule:

Allowed:
192.168.1.0/24

Incoming request:

192.168.1.50

Firewall checks:

Is 192.168.1.50 inside 192.168.1.0 - 192.168.1.255 ?

YES

Request allowed.

---

Another request:

10.0.0.5

Firewall checks:

Is 10.0.0.5 inside 192.168.1.0/24 ?

NO

Request blocked.