Subnet Calculator
Compute IPv4 subnet details from CIDR notation.
About Subnet Calculator
CIDR notation (like 192.168.1.0/24) compresses an IP address and its subnet mask into one string, but planning firewalls, VLANs, and DHCP scopes requires the details behind it: network address, broadcast address, mask, wildcard, and the usable host range. This calculator derives all of them instantly as you type, entirely in your browser.
Quick reference: /24 gives 254 usable hosts (a classic LAN segment), /16 gives 65,534 (a large internal network), /30 gives exactly 2 (point-to-point links), /32 is a single host route. Usable hosts equal 2 raised to the power of host bits, minus the network and broadcast addresses — except /31 and /32, which special-case to 2 and 1 per RFC 3021.
Network engineers use this for ACL sizing, firewall rules (the wildcard mask row maps directly to Cisco syntax), and capacity planning. The math is pure bit arithmetic in JavaScript — no lookup tables, no server, works offline once loaded.
Frequently Asked Questions
Why do /31 and /32 show different host counts?
Normally two addresses are reserved (network and broadcast). RFC 3021 exempts /31 point-to-point links (both addresses usable), and /32 denotes a single host with no subnet structure.
What is a wildcard mask for?
It is the inverted subnet mask, used in Cisco ACLs and OSPF configurations. A /24 mask 255.255.255.0 becomes wildcard 0.0.0.255 — match any host in that range.
Does it support IPv6?
This calculator covers IPv4, which remains the dominant format in firewall rules and internal networks. IPv6 subnets rarely need calculation in practice — the host portion is almost always /64.