What Is CIDR Notation? Reading /24, /16 and Beyond
CIDR writes a network as an address plus a prefix length, like 10.150.0.0/16. The prefix says how many bits identify the network; the rest are for hosts.
The slash number
In 10.150.0.0/16, the /16 means the first 16 bits are the network. The remaining 16 bits give 65,536 addresses. Each step up in the prefix halves the block: /17 is 32,768, /24 is 256, /30 is 4.
Prefix vs. subnet mask
A prefix is just a shorter way of writing a subnet mask. /24 equals 255.255.255.0, /26 equals 255.255.255.192, and /30 equals 255.255.255.252.
Alignment matters
A CIDR block must start on a boundary that matches its size. 10.0.0.64/26 is valid because 64 is a multiple of 64; 10.0.0.50/26 is not. Planning tools enforce this so subnets never overlap.