Subnet Calculator Online — Free Subnet Calculator Tool

CodexFuse Team · 2026-11-06T08:06:06.117Z · 11 min read min read

Split any IPv4 network into equal subnets in seconds — counts, masks, usable ranges and the mistakes that take offices offline, with a worked example.

A subnet calculator is a tool that splits an IP network into smaller, equal sub-networks and tells you exactly how many addresses each one holds, where each subnet starts and ends, and which addresses you can actually assign to devices. You type in a network address and a CIDR prefix, pick the size you want to split into, and the calculator returns the network address, broadcast address, usable range and subnet mask for every piece.

If you have ever stared at a spreadsheet of IP ranges and wondered whether 192.168.4.0/24 overlaps with 192.168.4.128/25, this is the tool that ends the guessing. A single wrong number can take a whole office offline — usually the moment a new branch opens or a VPN tunnel refuses to come up because two sites share the same range.

What does a subnet calculator actually do?

A subnet calculator takes a network like 192.168.0.0/16 and divides it into equal smaller networks at a longer prefix, then lists the network address, broadcast address, first and last usable host, and subnet mask for each resulting subnet. It answers the three questions every network plan starts with: how many subnets do I get, how big is each one, and which addresses belong where.

The maths underneath is simple once you see it. The number of subnets is 2 raised to the difference between the new prefix and the base prefix, and the size of each subnet is 2 raised to 32 minus the new prefix. Split a /16 into /24s and you get 2⁸ = 256 subnets of 256 addresses each. Split a /24 into /26s and you get 4 subnets of 64 addresses each. The calculator does this arithmetic instantly and, just as importantly, lays out the ranges so you can spot overlaps before they become outages.

The CodexFuse subnet calculator works entirely in your browser. You enter the network address — 192.168.0.0, for example — and its base prefix, choose the longer prefix to split into, and click Calculate Subnets. It shows the subnet count, the size of each subnet, and the network, broadcast and usable range for each one, listing the first 12 subnets so the pattern is easy to scan.

Why do networks need subnetting at all?

Networks are subnetted to contain broadcast traffic, isolate departments or sites from each other, and make address space manageable — a flat network of thousands of devices becomes slow, hard to secure and nearly impossible to troubleshoot, while subnets keep each segment small and predictable. Subnetting is how a growing business keeps one department's problem from becoming everyone's problem.

Picture a business with 60 staff, a warehouse, and a guest Wi-Fi network, all on one flat /24. Every broadcast packet reaches every device, so one misbehaving printer or chatty IoT sensor makes the whole network feel sluggish. Worse, there is no boundary: a compromised laptop on guest Wi-Fi can reach the machine that runs payroll.

Split that same space into /26 subnets and the picture changes. Staff get one subnet, the warehouse another, guests a third, and servers a fourth. A broadcast storm in one segment stays in that segment. A firewall rule between subnets can block guest devices from reaching internal systems entirely. And when something breaks, you know which segment to look at first instead of hunting across 254 addresses.

How do you split a network into subnets?

To split a network, enter the network address and base prefix, choose a longer new prefix, and calculate: the difference between the two prefixes decides how many subnets you get, and the new prefix decides how many addresses each subnet holds. Start from the number of hosts each segment actually needs, add room to grow, then pick the smallest prefix that fits.

Here is the practical sequence:

• Count the hosts per segment: list every device that needs an address in that subnet — workstations, printers, phones, cameras — then add at least 25% headroom for growth.
• Pick the prefix that fits: a /26 holds 62 usable hosts, a /25 holds 126, and a /24 holds 254, so match the prefix to your largest segment and use it for all of them to keep the plan uniform.
• Run the calculation: put the base network and the new prefix into the subnet calculator and check the ranges it returns against your list.
• Document the map: write down which subnet belongs to which department or site, and note the gateway address for each — usually the first usable host.
• Check for overlap: before applying anything, confirm no two subnets share an address; overlapping ranges are the single most common cause of mysterious "sometimes it works" network faults.

A worked example: one office, three floors

Take a fictional company, Northgate Dental Group, with three floors in one building and a single network, 192.168.0.0/16, to carve up. The IT lead needs a subnet per floor plus one for servers, and each floor has around 40 devices. A /26 gives 62 usable hosts per subnet — enough for 40 devices today with room for 22 more — so the new prefix is 26.

Entering 192.168.0.0 with base prefix 16 and new prefix 24 would give 256 subnets, but the lead only needs four, so they work from the top of the range instead: the calculator shows subnet 1 as 192.168.0.0/26 with usable hosts 192.168.0.1–192.168.0.62 and broadcast 192.168.0.63, subnet 2 as 192.168.0.64/26 (usable .65–.126), subnet 3 as 192.168.0.128/26 (usable .129–.190), and subnet 4 as 192.168.0.192/26 (usable .193–.254), all with mask 255.255.255.192. Ground floor takes subnet 1, first floor subnet 2, second floor subnet 3, and the server room subnet 4 with its gateway at 192.168.0.193. The whole plan fits on one page, every address is accounted for, and nothing overlaps.

Which subnet sizes should you use?

The right subnet size is the smallest prefix that comfortably holds the segment's devices plus growth — a /30 for a point-to-point link, a /26 or /25 for a department, and a /24 for a whole site — because oversized subnets waste addresses and undersized ones force a painful re-addressing later. The table below covers the sizes most small and mid-sized networks actually use.

Two edge cases catch people out. A /31 is the special point-to-point case defined by RFC 3021: it has just 2 addresses and both are usable, with no separate network or broadcast address — the calculator handles this the RFC 3021 way. A /32 is a single host address, used for loopbacks and specific host routes, not for a subnet with devices on it.

What are the common subnetting mistakes?

The most common subnetting mistakes are overlapping ranges, forgetting that the network and broadcast addresses are unusable, choosing a prefix with no room to grow, and documenting nothing — each one turns a five-minute calculation into hours of troubleshooting later. They are all avoidable, and they all show up in the same way: devices that work intermittently and nobody can explain why.

• Overlapping subnets: two segments given the same address space, usually because someone copied a range by hand instead of calculating it — always verify ranges with a calculator before applying them.
• Counting the network and broadcast addresses as hosts: planning 64 devices into a /26, then discovering only 62 can connect and the last two have nowhere to go.
• No growth headroom: sizing a subnet to today's exact headcount, so the first new hire or new printer forces a re-addressing of the whole segment.
• Undocumented ranges: a subnet plan that lives only in one person's head — when they are on holiday, nobody knows which range belongs to the warehouse.
• Mixing private ranges across sites: two offices both using 192.168.1.0/24, which breaks the VPN tunnel between them until one side is renumbered.

When you need to check a single address rather than split a whole network, the companion CIDR calculator works out the mask, range and host count for one CIDR block, and the CIDR to IP range guide shows how to turn one notation into the full address list. For the wider picture — keeping the servers behind those subnets patched, monitored and backed up — the VPS hosting guide for small business covers what managed hosting actually removes from your plate.

What this subnet calculator does not do

Honesty matters here, because a tool that overpromises is worse than no tool. This calculator splits an IPv4 network into equal subnets — it does not compute wildcard masks or binary notation, it does not scan or read your live network (you type the address yourself), and it is IPv4 only, so IPv6 subnets need a different calculation entirely. It also splits into equal-sized subnets only; if you need variable-length subnetting with different sizes per segment, calculate each block separately and check the ranges do not overlap. For a single IP's mask, wildcard and range, use the CIDR calculator instead.

Frequently Asked Questions

How do I split 192.168.0.0/16 into /24 subnets?

Enter 192.168.0.0 with base prefix 16 and new prefix 24 in the subnet calculator. You get 256 subnets of 256 addresses each: subnet 1 is 192.168.0.0/24 (broadcast 192.168.0.255, usable 192.168.0.1–192.168.0.254, mask 255.255.255.0) and subnet 256 is 192.168.255.0/24.

How many usable hosts are in a /30 subnet?

Two. A /30 has 4 addresses total: for 192.168.1.0/30 the network is .0, the broadcast is .3, and the usable hosts are .1 and .2. A /31 is the special point-to-point case (RFC 3021) with 2 addresses, both usable, and a /32 is a single host.

What is the subnet mask for /24, /26 and /16?

/24 is 255.255.255.0, /26 is 255.255.255.192, and /16 is 255.255.0.0. The mask has 1-bits for the prefix length and 0-bits after: /24 means the first 24 bits are the network, leaving 8 host bits, so 2⁸ = 256 addresses.

Can I use this subnet calculator for IPv6?

No — this calculator is IPv4 only. IPv6 subnetting works on the same prefix idea but with 128-bit addresses and very different block sizes, so it needs a dedicated IPv6 tool. Everything on this page, including the masks and host counts, applies to IPv4 networks.

Why does my VPN fail when both offices use 192.168.1.0/24?

Because the two networks have identical address ranges, so traffic meant for the other office is routed locally instead of across the tunnel. One side must be renumbered to a different range — plan the new subnets with the calculator first, then migrate one office's devices to the new range before bringing the tunnel up.

Do I need to install anything to use the subnet calculator?

No. It runs entirely in your browser, free, with no install and no sign-up. Type the network address and prefixes, click Calculate Subnets, and read the results — the first 12 subnets are listed with their network, broadcast and usable ranges.

Plan the network, then stop managing the infrastructure

A subnet calculator turns a job that used to take a spreadsheet and a prayer into a two-minute check — but the address plan is only the easy part. The servers behind those subnets still need patching, monitoring and backups, and someone awake at 3am when they go down.

Move it to CodexFuse. Bring your hosting, VPS, business email, security and backups onto one fully managed platform that runs on credits — pay-as-you-go, no subscriptions, no lock-ins, no server babysitting. Plan your subnets with the free tool today, then get started with CodexFuse and let the infrastructure look after itself.