IP Subnet Calculator

Network address, broadcast address, usable host range, and subnet mask from any IPv4 + CIDR

Subnetting — dividing an IPv4 network address space into smaller sub-networks — requires calculating the network address, broadcast address, usable host range, and subnet mask from an IP address and CIDR notation, and getting the math wrong produces a network configuration that either wastes address space or doesn't actually cover the intended range of devices.

Why CIDR notation determines everything else

An IP address paired with CIDR notation (like /24) specifies exactly how many bits of the address are reserved for the network portion versus the host portion — this single number determines the entire subnet structure: the network address (the lowest address in the range, identifying the subnet itself), the broadcast address (the highest address, reserved for broadcasting to all devices on the subnet), the usable host range (every address between network and broadcast, available to actually assign to devices), and the subnet mask (the same network/host split expressed in dotted-decimal form instead of CIDR notation). Each of these is mathematically derived directly from the CIDR prefix length — changing the CIDR number changes the size of the entire subnet and all four of these derived values together, not independently.

A worked example

Two subnets with the same base IP address but different CIDR prefix lengths (say, /24 versus /25) have genuinely different usable host ranges, network addresses, and broadcast addresses — a smaller CIDR number (fewer network bits) means a larger subnet with more usable host addresses, while a larger CIDR number means a smaller subnet with fewer, which is exactly why choosing the right CIDR prefix length for an intended device count matters before finalizing a network design.

How this connects to your other technical calculations

For working with network bandwidth and transfer estimates once a subnet is configured, the Bandwidth Calculator handles a related but distinct networking calculation — data transfer time rather than address space allocation.

Common mistakes

Choosing a CIDR prefix length without first counting how many actual devices need addresses on the subnet is a common planning mistake — a subnet sized too small runs out of usable addresses as devices are added, while one sized too large wastes address space that could have been allocated to other subnets, and getting this right from the start avoids a disruptive re-addressing later.