CIDR Calculator

CIDR (Classless Inter-Domain Routing) is the modern way of writing IP address ranges. Instead of the old A/B/C class system, a CIDR block is written as an address followed by a slash and the number of network bits — e.g. 10.0.0.0/16 means "the 65,536 addresses from 10.0.0.0 through 10.0.255.255". CIDR underlies every modern routing decision: BGP advertisements, AWS VPC subnets, Kubernetes pod CIDRs, firewall rules, and VPN tunnels.

This calculator takes a CIDR block and tells you the network address, broadcast address, first and last usable hosts, total host count, subnet mask, wildcard mask, and binary representation. It also supports IPv6 and lets you carve a block into smaller subnets.

1. Paste a CIDR block such as 10.0.0.0/24 or 2001:db8::/48 into the input.
2. Read the breakdown: network range, broadcast, first/last host, mask and binary form.
3. Use the subnet builder to split the block into smaller equal-sized chunks.
4. Copy any of the values for use in your firewall rule or routing table.

• Designing AWS VPC subnets without overlapping with on-prem ranges.
• Writing firewall rules with the right wildcard mask.
• Allocating non-overlapping CIDRs across Kubernetes clusters.
• Sanity-checking that a /16 contains a /24 you are about to advertise.