About the CIDR Calculator
CIDR notation compresses a subnet definition into an address and prefix length — 10.0.0.0/24 means the first 24 bits identify the network, leaving 8 bits for hosts. This calculator expands any CIDR into its full picture: network and broadcast addresses, usable host range, subnet and wildcard masks, and lets you split a block into smaller subnets or merge adjacent ones.
It is built for the daily VPC and cluster design questions: how many pods fit in this block, do these two subnets overlap, what /27s can I carve from this /24 — answered precisely instead of by mental arithmetic at 2 a.m. during an incident.
Frequently asked questions
How many usable hosts does a subnet have?
2^(32 − prefix) minus 2 — the network address and broadcast address are reserved. A /24 gives 254 usable hosts, a /27 gives 30. Cloud providers reserve more: AWS takes 5 addresses per subnet (first four + last), so an AWS /24 yields 251. Point-to-point /31 links (RFC 3021) are the exception with both addresses usable.
What is a wildcard mask and when do I need it?
A wildcard mask is the bitwise inverse of the subnet mask — 0.0.0.255 for /24. Cisco ACLs and OSPF configuration use wildcard masks instead of subnet masks, so misreading one as the other is a classic source of overly permissive firewall rules.
Why do overlapping CIDRs matter so much in cloud networking?
VPC peering, VPN tunnels, and Kubernetes pod ranges all fail — sometimes silently with asymmetric routing — when address spaces overlap. Once workloads run in an overlapping range, renumbering is painful, so checking candidate CIDRs against every connected network before allocation is far cheaper than fixing it later.
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