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IPv4 Subnet Calculator Guide

Work out the network, broadcast, mask and host range for a CIDR block.

An IPv4 address looks like 192.168.1.10. A subnet is a group of addresses, and a suffix such as /24 tells you how large the group is. IPv4 Subnet Calculator shows the group containing the address you enter.

Read one small example

The tool starts with 192.168.1.0/24 in CIDR block. CIDR is the address-plus-suffix notation. Look for these results:

  • Network: 192.168.1.0 — the start of this block.
  • Broadcast: 192.168.1.255 — the last address in this block.
  • Netmask: 255.255.255.0 — another way to write what /24 means.
  • First host and Last host: 192.168.1.1 and 192.168.1.254.
  • Total addresses: 256; Usable hosts: 254 under the ordinary network-and-broadcast rule.

Now type 192.168.1.10/24. The Network should still be 192.168.1.0: .10 lives inside the same block. This is why the result can start with a different address from the one you typed. The CIDR row shows the block’s starting address and prefix.

If you see an error

An IPv4 address must have four numbers from 0 to 255, separated by dots. A prefix must be from /0 to /32. Try 192.168.1.10/24 first, then change one part at a time. The calculator does not check whether your router, firewall, or cloud provider will actually let you use the range.

Where your input goes

Processing runs in your browser. The tool does not upload your input to ilham.dev.

For more background, read Network Debugging Notes.

Questions you might have

Why is the usable host count two less than the total?
For ordinary IPv4 subnets, the first address is reserved as the network address and the last as broadcast, so the tool subtracts two. Watch out: this tool reports 0 usable hosts for /31 and /32. That count is not a full explanation of those special cases: /31 can be used for a point-to-point link, and /32 names a single address. Check your network’s rules before treating the count as a deployment decision.
Can it plan a VLSM layout?
Not directly. Use it to check one block at a time, or the range expander to see which CIDR blocks cover an arbitrary range exactly.