Zellio.io

Subnet Calculator

Calculate network, broadcast, host range and split any CIDR block.

Runs locallyWorks offlineShare link carries settings, never your data
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Frequently asked questions

Is my network data sent anywhere?

No. Everything runs locally in your browser using standard web APIs — your text, files and inputs are never uploaded to a server, so the tool works even offline once the page has loaded.

What is the difference between total and usable hosts?

Total addresses include the network and broadcast addresses, which cannot be assigned to hosts. Usable hosts subtract those two — except for /31 (a point-to-point link with 2 usable addresses per RFC 3021) and /32 (a single host). In a typical /24 network (192.168.1.0/24), you have 256 total addresses, but only 254 are usable for actual devices (subtracting .0 for network and .255 for broadcast). Understanding this distinction is critical for network planning, ensuring you allocate subnets large enough to accommodate all the devices you need while accounting for overhead.

Can I enter a netmask instead of a CIDR prefix?

Yes. You can type 10.0.0.5/24, 10.0.0.5 255.255.255.0, or a bare IP (which is treated as /32). The calculator accepts all three.

Pro tips

  • Plan subnets from the largest requirement down. Allocating small blocks first fragments the range and forces a renumber later.
  • Leave headroom — a /24 that is 90% full on day one is a migration waiting to happen, and renumbering a live network is far more expensive than reserving space.
  • Check the wildcard mask when writing access control lists; several vendors expect it rather than the subnet mask, and the two are inverses.
  • Use the split view to confirm that child subnets tile the parent exactly, with no overlap and no gap, before configuring anything.
  • Write allocations down in CIDR rather than as ranges. '10.0.4.0/22' is unambiguous where '10.0.4.0 to 10.0.7.255' invites an off-by-one.

About Subnet Calculator

Usable hosts always come to two below the total: the first address names the network and the last is the broadcast address, and neither can be assigned. That arithmetic is why a /30 yields only two usable addresses.

A subnet calculator turns an IPv4 address and its CIDR prefix (or netmask) into everything you need to plan or troubleshoot a network: the network and broadcast addresses, the first and last usable host, the total and usable host counts, the wildcard mask and the address class. Enter something like 192.168.1.10/24 — or a netmask such as 255.255.255.0 — and every value updates instantly.

It also splits a block into equal subnets, which is the core of VLSM (variable-length subnet masking) design. Slide to a longer prefix to see how a /24 divides into /26s or /30s, complete with each child network's usable range and broadcast address.

Prefix length is the whole game, and the pattern is worth internalising: every bit added to the prefix halves the range. A /24 holds 254 usable addresses, a /25 holds 126, a /26 holds 62, and a /30 holds exactly two — which is why point-to-point links between routers are conventionally allocated a /30. Reading a prefix as 'how many halvings from a full octet' is faster than counting addresses.

Which range you allocate from is a decision worth making deliberately, because it is expensive to revisit. Three ranges are reserved for internal use — 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16 — and home routers overwhelmingly default into the same corner of the last one. The consequence shows up the day two networks have to talk: a VPN between two sites that both chose 192.168.1.0/24 cannot route, because each end believes that range is local. Picking something unfashionable from 10.0.0.0/8 costs nothing now and avoids a renumber later.

Common use cases

  • Network engineers carving an allocation into per-site or per-VLAN subnets during a design review.
  • System administrators confirming whether two hosts are on the same subnet when troubleshooting traffic that will not route.
  • Cloud engineers sizing VPC and subnet CIDR blocks before creating them, since several providers will not let you resize afterwards.
  • Security teams translating a firewall rule between subnet-mask and wildcard-mask notation without transposing a digit.
  • Students working through subnetting exercises and wanting the intermediate values checked rather than just the answer.
How it comparesCommand-line tools such as ipcalc and sipcalc give the same numbers and assume a terminal and a memorised flag set. Compared with other web calculators, the split view is the part that earns its place — seeing a /24 divide into its children is the step where hand arithmetic usually goes wrong.