🧮 Subnet Calculator ☆

Overview

Subnet Calculator is a complete IPv4 subnetting tool for developers and network engineers. Enter any address like 192.168.1.10 with a CIDR prefix such as /24 and instantly get the network address, broadcast address, first and last usable host, usable host count, total addresses, subnet mask, wildcard mask, CIDR notation, IP class and scope (private, public, loopback, link-local, multicast). Special prefixes are handled correctly: /31 uses the RFC 3021 two-host rule and /32 returns a single address. Everything is calculated locally in your browser with pure JavaScript. No data is sent anywhere and the tool even works offline.

How subnetting math works

The prefix length splits the 32-bit address into network and host parts. The network address is the address with all host bits cleared; the broadcast address has all host bits set; the usable range runs from network + 1 to broadcast − 1. Host counts follow 2^(32−prefix) − 2, except for the special prefixes: /31 applies RFC 3021 (two usable addresses on point-to-point links) and /32 is a single host route. Masks are shown in dotted-decimal, wildcard and CIDR forms, and the scope check identifies private ranges (RFC 1918), loopback, link-local and multicast addresses.

Technical specifications

Property	Subnet calculator behavior
Input	IPv4 address with CIDR prefix (/0: /32)
Outputs	Network, broadcast, first/last host, host counts, masks, class, scope
Special prefixes	/31 RFC 3021 two-host rule, /32 single address
Scopes	Private, public, loopback, link-local, multicast
Processing	100% client-side JavaScript; works offline
Cost	Free, no account, no ads

Privacy: network data stays local

Internal addressing schemes reveal infrastructure layout, so calculations run entirely in the browser. Nothing is transmitted or logged, and the tool works without a connection.

Subnetting tips

• Remember the /24 to /30 shortcuts: 254, 126, 62 and 2 usable hosts.
• Reserve the network and broadcast addresses in every plan.
• Use /31 on point-to-point links to avoid wasting two addresses.
• Overlap-check ranges before adding VLANs or VPN pools.
• Document the plan alongside VLAN IDs and gateway addresses.

Related: browse the developer tools for DNS, IP, whois and header checks.

Network, broadcast and usable host addresses

Every IPv4 subnet has three addresses that behave differently from the rest. The network address has all host bits set to zero and identifies the subnet itself; it cannot be assigned to a device. The broadcast address has all host bits set to one and reaches every host in the subnet at once, so it is also unavailable for assignment. Everything in between is usable for hosts. That is why a /24 has 256 addresses but only 254 usable ones, and a /25 has 126, not 128. Two special cases surprise people: a /31 contains exactly two addresses, and RFC 3021 allows both to be used for point-to-point links, while a /32 represents a single host and is normally used for loopback addresses or host routes. The subnet mask and the CIDR prefix are two notations for the same information, /26 and 255.255.255.192 describe an identical boundary. When planning, start from the number of hosts you need and round up to the next power of two, then add two for the network and broadcast addresses. Also remember that private ranges (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16) are not routed on the public internet, and that the default gateway must sit inside the same subnet as the hosts it serves.