CIDR Notation Explained

1. Why CIDR was Needed

In the early days of the internet, IP addresses were assigned in rigid "Classes" (A, B, and C). This was incredibly wasteful. A company needing 300 addresses would have to take a Class B block (65,000 addresses), wasting nearly 99% of the space.

CIDR was introduced in 1993 to allow for "classless" addressing, where network blocks of any size could be created.

2. How to Read Slash Notation

The number after the slash represents the number of "1" bits in the binary subnet mask.

3. The Binary secret

IP addresses are 32 bits long. A /24 means that the first 24 bits belong to the network, and the remaining 8 bits (32 - 24 = 8) belong to the hosts.

Since 2^8 = 256, a /24 network has 256 addresses (minus 2 for network and broadcast IDs, leaving 254 usable).