~/ ipcalc

Subnet & Netmask Calculator

Enter any IPv4 address with a CIDR prefix or a dotted-decimal netmask. All math runs in your browser — no data is sent to a server.

// results

Network address

—

The base address of the subnet, used to identify the network itself. Cannot be assigned to a host.

Broadcast address

—

Sends to every host on the subnet. Reserved — cannot be assigned. Not used in /31 (RFC 3021) or /32.

First usable host

—

Lowest address you can assign to a device on the subnet.

Last usable host

—

Highest assignable address.

Usable hosts

—

Total addresses minus network + broadcast. /31 has 2 usable, /32 has 1.

Total addresses

—

2^(32−prefix). Counts every address in the block, usable or not.

Netmask

—

Dotted-decimal form. Marks which bits identify the network vs the host.

Wildcard mask

—

Inverted netmask (~mask). Used in Cisco ACLs and OSPF.

IP class

—

Historic classful range (A/B/C/D/E). Classless routing (CIDR) supersedes it, but the label is still common in docs.

IP type

—

Private (RFC 1918), public, loopback, link-local, CGNAT (RFC 6598), multicast or reserved.

Binary

—

Each octet rendered in base 2. Useful to visualize which bits are masked by the prefix.

Programmatic access

Same calculation via JSON API: /api/subnet?ip=192.168.1.0&cidr=24

What this subnet calculator does

A subnet calculator takes an IPv4 address and either a CIDR prefix length (like /24) or a dotted-decimal netmask (like 255.255.255.0) and returns every derived property of that block: the network address, the broadcast address, the first and last assignable host, the total number of addresses and the wildcard mask. It is the daily-driver tool for anyone working on IP planning, ACLs, firewall rules, BGP advertisements or static routing.

CIDR notation (Classless Inter-Domain Routing, RFC 4632) replaced the old classful A/B/C system in 1993. Instead of fixed octet boundaries, CIDR allows any prefix length from /0 to /32, so subnets can be sized to fit the number of hosts you actually need. The prefix counts the leading 1 bits in the netmask: a /24 means the first 24 bits are the network identifier and the remaining 8 bits address individual hosts, giving 256 total addresses and 254 usable.

Two prefixes break the usual "subtract 2 for network and broadcast" rule. A /31 (RFC 3021) is meant for point-to-point links and has two usable addresses with no broadcast. A /32 represents a single host route, common in loopback interfaces and BGP next-hops. This netmask calculator handles both correctly.

All computations are performed locally in your browser using bitwise operations on 32-bit integers — no inputs ever leave your device. If you need machine-readable output for scripts or LLM agents, the same logic is exposed at /api/subnet (JSON) and /api/subnet.md (Markdown).