IPv4 Subnet Calculator Online (CIDR) — Free
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Slicing networks by hand is where address math goes wrong. Enter an IPv4 address with or without a mask — 192.168.0.1/24, 10.0.0.0/8 — and read the full breakdown: netmask, network address, wildcard mask, first and last usable hosts, broadcast address, block size, and IP class. Step to the previous or next block with one click.
How it works
- 1
Enter an address or CIDR
Both plain addresses and IP/mask notation are parsed.
- 2
Read the breakdown
Netmask, host range, broadcast address, size, and class appear instantly.
- 3
Walk adjacent blocks
Previous/next buttons shift the block by its own size.
About this tool
What the calculator works out
Enter an IPv4 address and a subnet mask (or CIDR prefix), and the tool computes the network address, broadcast address, first and last usable hosts, total and usable host counts, and the wildcard mask — the full anatomy of the subnet in one pass.
The questions it answers
How many hosts fit in this subnet? Is this address inside that network? What mask do I need for at least 100 hosts? Network planning, firewall rules, and route tables all start from these answers — and the arithmetic is exactly the part humans get wrong.
CIDR is the modern notation
/24 means 24 one-bits in the mask — 255.255.255.0 — leaving 254 usable hosts. The tool accepts both notations and shows the equivalence, which is how the slash notation stops being mysterious: every CIDR value maps to one mask and one host count.
Frequently asked questions
Why are two addresses in every subnet unusable?
The network address (all host bits zero) names the subnet itself, and the broadcast address (all host bits one) reaches every host. Both are reserved, so a /24 holds 254 usable hosts, not 256.
What mask do I need for N hosts?
Count the host bits: 2^(bits) minus 2 must cover N. The calculator works backward from any mask — adjust the prefix and watch the usable count until it fits.
Can subnets overlap?
They can — and when they do, routing becomes ambiguous. Compare the network addresses of neighboring subnets; this calculator shows each one's range precisely.