IPv4 Subnet Calculator

You do not need to enter the network address itself. Enter any address from the subnet, select its CIDR prefix, and the calculator will move the host bits to the correct boundaries. For example, 192.168.1.130/26 belongs to 192.168.1.128/26 even though the address you entered is not the first address in that block.

This is useful when checking an interface configuration, reading a firewall rule, diagnosing why two devices cannot communicate directly, verifying a DHCP scope, or studying subnetting. The tool analyzes one existing CIDR block; it does not divide a parent network into multiple new subnets or build a VLSM allocation plan.

A subnet result is a set of boundaries, not just a mask. The CIDR block identifies the complete network, the usable range shows the addresses normally available to endpoints, and the wildcard mask expresses the inverse of the subnet mask for systems that use inverse matching.

The table below explains what each displayed value means and where it is commonly used.

Prefixes such as /8, /16, and /24 align neatly with octet boundaries, but most real subnet sizes do not. A /23 crosses two adjacent values in the third octet, while a /26 divides the final octet into blocks of 64 addresses. Looking only at the dotted decimal numbers can hide that boundary.

The binary view exposes it. The subnet mask contains one bit for every network bit and zero for every host bit. A bitwise AND between the entered IP address and that mask produces the network address. Replacing every host bit with one produces the upper boundary, which is the broadcast address for ordinary prefixes from /0 through /30.

This is also why two similar-looking addresses may be in different networks and two addresses with different third octets may still share a /23 subnet.

The familiar usable-host formula subtracts the network and broadcast addresses. Applying it blindly to every prefix would produce zero usable addresses for /31 and a negative result for /32.

RFC 3021 allows both addresses in a /31 to operate as endpoints on a point-to-point link, without a traditional directed broadcast address. A /32 identifies one address and is commonly used as a host route, loopback route, firewall object, or exact route match.

RFC 1918 defines three private-use blocks: 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16. An address outside those blocks is not automatically an ordinary public address.

IPv4 also contains shared CGNAT space, loopback addresses, link-local addresses, documentation ranges, benchmarking space, multicast space, and other protocol reservations. The calculator identifies the major categories so an address such as 127.0.0.1 is not incorrectly described as public merely because it is not RFC 1918 private.

A “Public unicast” result means the address is not inside one of the special-use blocks recognized by this calculator. It does not prove that the address is currently allocated, announced through BGP, reachable from your location, or assigned to a particular organization.

For an ordinary /24, the calculator reports 256 total addresses and 254 traditionally usable host addresses. That is the protocol-level calculation after reserving the network and broadcast addresses.

Cloud platforms, managed networks, and individual organizations may reserve additional addresses for gateways, DNS services, platform infrastructure, or future use. When planning an AWS, Azure, Google Cloud, Kubernetes, VPN, or provider-managed network, treat this calculator as the CIDR boundary check and confirm the provider-specific usable count separately.

The same caution applies when planning growth. A subnet that supports the exact number of devices required today leaves no room for routers, load balancers, failover addresses, temporary workloads, or later expansion.

The prefixes below cover the subnet sizes most often encountered in LANs, VLANs, cloud networks, firewall rules, point-to-point links, and host routes. Total addresses always follow 2 raised to the number of host bits; usable addresses change for /31 and /32.