IPv4 Subnet Calculator

A Worked Example: Splitting an Office Network

Suppose you administer 192.168.1.0/24 for a 120-person office and want to separate staff from guest Wi-Fi. Entering 192.168.1.0 with prefix /25 returns network 192.168.1.0, broadcast 192.168.1.127, and 126 usable hosts — the staff half. The second /25 (192.168.1.128/25) becomes the guest network.

Every field in the output has a job: the network address identifies the subnet in routing tables, the broadcast address is where broadcast packets land, and the usable range is what you hand out via DHCP. The two reserved addresses (network and broadcast) are why a /24 yields 254 usable hosts, not 256.

Common Subnetting Mistakes

• Assuming the network address must end in .0 — that is only true for /24 and larger; a /26 can start at .0, .64, .128 or .192.
• Forgetting that /31 links have no broadcast address (RFC 3021) and /32 is a single host route.
• Handing the broadcast address to a device and then debugging mysterious ARP problems.
• Overlapping subnets when dividing by hand — always verify the usable range of the previous block before starting the next one.

Working Out a Subnet by Hand

To find the network address, apply the mask to the IP with a bitwise AND; the broadcast address is the network address with every host bit set to 1. For 192.168.10.77/26, the block size in the last octet is 64, so the network is 192.168.10.64 and the broadcast is 192.168.10.127.

Usable hosts equal 2^(host bits) minus 2. The wildcard mask is the inverse of the subnet mask, which is the form used in router access lists. When planning, start with the largest subnet first and leave room to grow.