IPv4 Subnet Calculator
A simple subnet tool: calculate IPv4 CIDR details, split a parent prefix into equal subnets, and optionally find the next available subnet inside a parent block using a pasted used-subnets list. (See also: → VLAN Planner | → Communication Matrix)
| # | Subnet | Usable hosts | Copy |
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"Available" means: inside your parent prefix, aligned to the target size, and not overlapping any used subnet you paste.
A subnet mask defines which portion of an IP address identifies the network versus the host. CIDR (Classless Inter-Domain Routing) notation expresses this as a prefix length after a slash – for example, /24 means 24 bits for the network portion, leaving 8 bits for hosts (256 addresses, 254 usable). The subnet mask 255.255.255.0 equals /24. Smaller prefix numbers (/16, /8) create larger networks with more hosts, while larger numbers (/28, /30) create smaller subnets.
The formula is 2^(32-prefix) - 2 for usable hosts (subtracting network and broadcast addresses). A /24 has 254 usable hosts (256-2), a /25 has 126 usable hosts (128-2), and a /26 has 62 usable hosts (64-2). For point-to-point links, /31 subnets provide exactly 2 addresses without wasting IPs on network/broadcast. A /32 represents a single host address, commonly used for loopbacks or host routes.
The network address is the first address in a subnet (all host bits set to 0) and identifies the subnet itself – it cannot be assigned to a device. The broadcast address is the last address (all host bits set to 1) and is used to send packets to all hosts in the subnet simultaneously. For example, in 192.168.1.0/24, the network address is 192.168.1.0 and broadcast is 192.168.1.255. Usable host addresses are everything between these two.
To split a network into equal subnets, you need to borrow bits from the host portion. The number of subnets must be a power of 2 (2, 4, 8, 16, etc.). For example, splitting a /24 into 4 equal subnets requires borrowing 2 bits (2²=4), resulting in four /26 subnets. Each /26 contains 64 addresses (62 usable). Use this calculator's split function to automatically generate the subnet boundaries and see the resulting address ranges.
Finding available address space requires checking for overlaps with existing allocations. Enter your parent prefix (e.g., 10.0.0.0/16), specify your target subnet size (e.g., /24), and paste your list of already-used subnets. The calculator finds the first available subnet that doesn't overlap with any existing allocation and is properly aligned to the target size. This prevents IP conflicts and helps maintain organized addressing schemes.
RFC 1918 defines three private address ranges: 10.0.0.0/8 (16 million addresses), 172.16.0.0/12 (1 million addresses), and 192.168.0.0/16 (65,000 addresses). These are not routed on the public internet and can be used freely within private networks. Use private addresses for internal networks, OT systems, and any devices that don't need direct internet access. For industrial networks, 10.x.x.x is often preferred due to its large address space allowing logical organization by site, zone, or function.
Our IP address management (IPAM) helps you keep IP documentation accurate and avoid conflicts, especially in long-lived OT networks.
