What CIDR means
CIDR combines an IP address with a prefix such as /24. The prefix tells you how many bits belong to the network.
Instructions / Networking
IP subnet calculator instructions
Use this guide to understand CIDR notation, subnet masks, host counts, and AWS VPC subnet rules.
Subnet mask and broadcast
The subnet mask shows which part of the address is the network. The broadcast address is the last IP in the block for standard IPv4 subnetting.
Usable host count
Standard IPv4 subnets usually subtract network and broadcast addresses. /31 and /32 are special cases and are handled explicitly.
AWS VPC subnet rules
AWS reserves the first four IP addresses and the last IP in every subnet. For AWS VPC design, use subnet sizes from /16 through /28.
Example calculation
Input: 10.0.1.15/24
Canonical CIDR: 10.0.1.0/24
Subnet mask: 255.255.255.0
Usable addresses in standard mode: 254
Common mistakes
- Confusing /24 with 24 usable IPs.
- Forgetting network and broadcast addresses.
- Forgetting AWS reserved addresses.
- Choosing a subnet size that is too small for future growth.
- Using ranges that overlap Docker, VPN, or peering networks.
Private IP ranges to remember
- 10.0.0.0/8
- 172.16.0.0/12
- 192.168.0.0/16
Limitations
This calculator currently focuses on IPv4. It does not split one CIDR into multiple subnets yet, and it does not check for overlap between two separate CIDRs.