IPv4 Subnet Calculator

Enter an IPv4 address and CIDR prefix (or pick a subnet mask) to instantly calculate the network address, broadcast, usable host range, wildcard mask, binary representation, and address class.

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Understanding Subnetting

Subnetting splits a single IP network into smaller sub-networks by extending the prefix length (CIDR). The network address is the IP with all host bits zero; the broadcast address has all host bits one. Everything in between is a usable host. Subnetting matters because it limits broadcast domains (improving performance), enables granular firewall and ACL rules, and reduces blast radius during a breach. A /24 gives 256 addresses with 254 usable; a /30 gives 4 addresses with 2 usable — the typical point-to-point link. The wildcard mask (inverse of the subnet mask) is what Cisco ACLs and OSPF use instead of a regular mask.

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