CIDR subnetting guide: prefixes, masks, and ranges
Understand IPv4 CIDR notation, translate prefixes into subnet masks, and verify network, broadcast, and usable host ranges.
A prefix defines the network bits
CIDR writes an address and prefix together, such as 192.168.10.42/24. The /24 says that the first 24 bits belong to the network, leaving eight bits for addresses inside that subnet.
For common IPv4 networks, /24 maps to 255.255.255.0, /16 maps to 255.255.0.0, and /8 maps to 255.0.0.0.
192.168.10.42/24
network: 192.168.10.0
broadcast: 192.168.10.255Calculate ranges from the block size
A /24 has 256 total addresses. A /30 has four. In traditional IPv4 subnetting, the network and broadcast addresses are reserved, so usable hosts are usually two fewer. Always check the platform’s rules for special point-to-point or cloud networking cases.
10.0.0.9/30
network: 10.0.0.8
hosts: 10.0.0.9–10.0.0.10
broadcast: 10.0.0.11Document intent as well as ranges
A correct CIDR block can still overlap another network or be too small for growth. Record its purpose, environment, owner, and allocation boundary alongside the calculated range.