Developer Tools
Calculate Subnet Details
Calculate subnet from your inputs with a quick browser-based result.
Direct Answer
Use the Subnet Calculator when planning or debugging IPv4 networks and you need the usable range, subnet mask, network address, and broadcast address.
Planning Subnets From a Host or Subnet Count
This calculator is often used the opposite way from a plain CIDR lookup: instead of starting with a prefix, you start with a requirement, like "I need 4 subnets" or "I need 50 hosts per subnet," and work out what mask satisfies it. That process is commonly called VLSM (Variable Length Subnet Masking) when different subnets in the same network need different sizes.
- Round a required host count up to the next power of two, then subtract 2 for the network and broadcast addresses to find the smallest prefix that fits.
- 50 required hosts rounds up to 64 addresses, which needs 6 host bits, giving a /26 mask (62 usable hosts).
- Splitting one block into equal subnets means borrowing bits from the host portion; each borrowed bit doubles the subnet count and halves the hosts per subnet.
- VLSM lets different subnets in the same allocation use different prefix lengths, matched to each subnet's actual host requirement instead of one fixed size for all of them.
Common Subnetting Edge Cases
A handful of prefix lengths and rounding situations come up constantly when subnetting, and getting them wrong is the most common source of "my subnet math doesn't match the calculator" confusion.
- A /30 gives only 2 usable hosts after subtracting network and broadcast, which is the classic size for a point-to-point router link.
- A /31 has no usable hosts under the classic rule, but RFC 3021 allows both addresses in a /31 to be used on point-to-point links with no network or broadcast reserved.
- You can never use the network address or the broadcast address as a host address; both are reserved to identify the subnet and to reach every host in it at once.
- IPv6 subnetting doesn't reserve a broadcast address at all, so host counts for equivalent prefix sizes are computed differently than in IPv4.
How to Use Subnet Calculator
- Fill in the input fields with your values.
- The result calculates automatically as you type.
- For spreadsheet work, switch to Bulk paste and paste rows copied from Excel or Google Sheets.
- Copy the result column and paste it back into your spreadsheet.
- Use the Copy result button to copy the output to your clipboard.
- Press Reset to clear all fields and start again.
Formula Reference
| Feature | Details |
|---|---|
| Purpose | Calculate a numeric answer from the values entered. |
| Single input | Numbers, dates, rates, or options depending on the calculator. |
| Bulk input | Rows copied from Excel, Google Sheets, CSV, or tab-separated columns. |
| Bulk output | A copyable result column or full table that can be pasted back into a spreadsheet. |
| Best for | Quick checks, homework, finance estimates, planning, everyday math, and spreadsheet cleanup. |
| Category | Developer Tools |
| Works on | Desktop, tablet, and mobile browsers |
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Useful Related Tools
Frequently Asked Questions
How do I calculate a subnet mask if I need 50 hosts per subnet?
Round 50 up to the next power of two, which is 64, requiring 6 host bits (2^6 = 64). That leaves 26 network bits, giving a /26 mask (255.255.255.192) with 62 usable hosts after subtracting the network and broadcast addresses.
How is the Subnet Calculator different from the CIDR Calculator?
The CIDR Calculator is generally used when you already have a CIDR string like 192.168.1.0/24 and want its range and host count. This tool is more often used in the reverse direction, planning what mask or prefix satisfies a required number of hosts or subnets, which is the core of VLSM design.
What is VLSM and when do I need it?
VLSM (Variable Length Subnet Masking) means using different subnet mask sizes within the same address block instead of one fixed size everywhere. It's needed when different parts of a network have very different host counts, such as a /30 for a router link and a /26 for an office LAN carved from the same block.
Why do I round a host count up to the next power of two?
The number of addresses in a subnet is always a power of two, since it's determined by how many host bits are available. Rounding 50 hosts up to 64 (2^6) is what finds the smallest subnet size that can actually contain the requirement.
How many equal subnets can I split a /24 into?
It depends on how many bits you borrow from the host portion; borrowing 2 bits gives 4 subnets, borrowing 3 bits gives 8 subnets, and so on, following powers of two. Each bit borrowed for subnets is a bit no longer available for hosts, so more subnets means fewer hosts per subnet.
What subnet mask gives exactly 30 usable hosts?
A /27 mask (255.255.255.224) provides 32 total addresses, which becomes 30 usable hosts after subtracting the network and broadcast addresses. That's the smallest standard prefix that covers a 30-host requirement.
Why does a /30 only give 2 usable hosts, and where is that used?
A /30 has 4 total addresses, and subtracting the network and broadcast addresses leaves exactly 2. That's intentionally just enough for a point-to-point link between two routers, without wasting a larger block on a connection that only ever needs two addresses.
What about a /31 - does it have any usable hosts?
Under the classic rule a /31 has zero usable hosts because both of its 2 addresses would be network and broadcast. RFC 3021 overrides that for point-to-point links, allowing both addresses to be assigned to the two endpoints with no network or broadcast address reserved at all.
What's the difference between network address, broadcast address, and usable range?
The network address is the first address in a subnet and identifies the subnet itself rather than any host. The broadcast address is the last address and is used to reach every host in the subnet at once. Everything between those two is the usable range that can be assigned to actual devices.
Why can't I assign the network or broadcast address to a device?
Both addresses have a reserved meaning at the network layer: the network address represents the subnet as a whole, and the broadcast address represents "every host in this subnet." Assigning either to a single device would create ambiguity for any router or switch trying to route or broadcast traffic correctly.
How do I split 192.168.1.0/24 into 4 equal subnets?
Borrowing 2 host bits from a /24 gives 4 subnets of /26 each, with 62 usable hosts per subnet. The four resulting blocks are 192.168.1.0/26, 192.168.1.64/26, 192.168.1.128/26, and 192.168.1.192/26.
What CIDR prefix is subnet mask 255.255.255.192?
255.255.255.192 has 26 consecutive 1 bits, which corresponds to a /26. That leaves 6 host bits, for 64 total addresses and 62 usable hosts.
Do usable host counts work the same in IPv6 subnetting?
No, IPv6 does not reserve a broadcast address the way IPv4 does, so a given prefix length in IPv6 does not lose 2 addresses to network and broadcast reservations. IPv6 subnets are also almost always sized much larger than IPv4 subnets, commonly using /64 regardless of actual host count.