CIDR (Classless Inter-Domain Routing) notation is the standard way to express IP address ranges. Understanding CIDR is essential for network design, subnetting, and IP address management.
Understanding CIDR Notation
CIDR notation combines an IP address with a suffix indicating the network prefix length. For example, 192.168.1.0/24 means the first 24 bits identify the network, leaving 8 bits for host addresses (256 addresses total).
The number after the slash indicates how many bits are fixed in the network portion. A higher number means a smaller network; a lower number means a larger network.
Common CIDR Blocks
Here are the most commonly used CIDR block sizes in IP leasing and network infrastructure.
CIDR Calculations
To calculate addresses in a CIDR block, subtract the prefix from 32 and use that as the exponent of 2. For example: /24 = 2^(32-24) = 2^8 = 256 addresses.
Addresses = 2^(32 - prefix)
Why CIDR Matters for IP Leasing
When leasing IPv4 addresses, you select a CIDR block size that matches your needs. Smaller blocks (/24) offer flexibility, while larger blocks (/22, /20) provide better per-IP pricing.
Most ISPs require a minimum of /24 for BGP announcements, making this the smallest practical block for independent routing.