TOOLS / CIDR CALCULATOR
CIDR / Subnet Calculator
Convert any IPv4 CIDR into network range, broadcast, usable host count, and netmask. All math happens in your browser — no requests leave your device.
What it does
CIDR (Classless Inter-Domain Routing) notation is how modern IP networks describe subnets. Every firewall rule, route, security-group entry, and ACL ultimately resolves to a network address and a prefix length. Our calculator takes any CIDR and returns the full breakdown: network address, broadcast, first/last usable host, netmask, wildcard mask (for Cisco ACLs), and binary representation. All math is local — your input never leaves the browser.
How to use it
- Enter any IPv4 CIDR (e.g. 10.0.0.0/24 or 192.168.1.50/22).
- Click "Calculate" — results appear instantly.
- Use "Network address" + "Broadcast address" to validate firewall ranges.
- Use "Wildcard mask" for Cisco-style ACLs (inverse of subnet mask).
- Binary mask helps when troubleshooting non-aligned subnets.
Common use cases
Firewall rule review
When auditing rules, paste each CIDR to confirm it covers what you think — broadcast / network mistakes are a common over-permission source.
Subnet planning
Plan VLAN sizes by checking host-count for each /22, /23, /24 candidate.
VPC / cloud networking
Validate AWS/GCP/Azure subnet allocations don’t overlap before deploying.
Wildcard ACL generation
Quickly get the wildcard mask for Cisco IOS / NX-OS ACLs (inverse netmask).
Frequently asked questions
What is CIDR notation? +
A /N suffix on an IP address indicating the number of network-bit prefixes. /24 = 24-bit network = 256 addresses; /16 = 65 536 addresses.
Why are /31 and /32 special? +
/31 is a 2-address subnet used for point-to-point links (RFC 3021) — no network or broadcast wasted. /32 is a single host (host route).
Does this support IPv6? +
Not yet. IPv6 CIDR uses the same /N syntax but addresses are 128 bits — the math is the same, but our current UI focuses on IPv4. We’ll add IPv6 in a future release.
What is the wildcard mask? +
The inverse of the subnet mask. /24’s subnet mask is 255.255.255.0; its wildcard is 0.0.0.255. Cisco ACLs use the wildcard for match patterns.
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Free for everyone, no signup required. Tool runs at /tools/cidr-calculator/ — bookmark or share.