How to Use an IP Subnet Calculator Free — CIDR & IPv4 (2026)

IP subnet calculations are needed whenever you configure routers, set up firewall rules, partition a network into VLANs, or troubleshoot connectivity issues. A subnet calculator takes an IP address and a prefix length (CIDR) or subnet mask and derives all the key values: network address, broadcast address, usable host range, and host count.

How to Use

1. Enter an IPv4 address (e.g. 192.168.1.0).
2. Choose CIDR notation and enter a prefix length (e.g. /24), or choose Subnet mask and enter it as a dotted-decimal (e.g. 255.255.255.0).
3. Click Calculate to see the full subnet breakdown.

How It Works

The subnet mask is a 32-bit number with consecutive 1s in the high bits and 0s in the low bits. A /24 prefix means 24 ones followed by 8 zeros: 11111111.11111111.11111111.00000000 = 255.255.255.0. The network address is the bitwise AND of the IP and mask. The broadcast address is the network address OR'd with the inverted mask (wildcard mask). Usable hosts = 2^(host bits) − 2, subtracting the network and broadcast addresses.

Common CIDR Values

/24 gives 254 usable hosts (a typical home or small office LAN). /16 gives 65,534 usable hosts. /30 gives 2 usable hosts and is used for point-to-point links between routers. /32 is a host route — exactly one IP. /0 covers the entire IPv4 address space.

Frequently Asked Questions

What is the wildcard mask?

The wildcard mask is the bitwise complement of the subnet mask. It is used in Cisco ACLs and OSPF configurations to specify which bits in an address must match. A /24 subnet has wildcard mask 0.0.0.255.

Why does /31 only show 2 usable hosts?

RFC 3021 allows /31 prefixes for point-to-point links, where both addresses are usable (no network or broadcast address needed). /32 gives a single host — itself.

Does this support IPv6?

Not currently. This tool calculates IPv4 subnets only.

Is this free?

Yes, entirely free with no signup required.