Quick start
- In IP address, enter four decimal octets from 0 to 255, such as 10.0.5.17.
- In Prefix length, enter a whole number from 0 to 32. Type 28 for /28, without the slash.
- Press Calculate subnet to see the network, subnet mask, wildcard mask, broadcast, usable range, and usable address count.
Best uses
Start here if one of these sounds like your job. The examples below show which inputs matter most.
- Find the network and broadcast address for an IPv4 CIDR block.
- Convert a prefix length such as /24 into a dotted decimal subnet mask.
- Check usable host range and address count.
- Practice subnetting for networking, security, or developer work.
What this calculator is solving
The Subnet Calculator helps developers, students, and network learners check IPv4 CIDR blocks without doing every binary step by hand.
Match each input label on the calculator to the dotted IPv4 address and the separate whole-number prefix, not a combined address/prefix string.
The formula in plain language
The prefix counts the network bits in a 32-bit IPv4 address. The remaining bits give 2^(32 - prefix) total addresses. The subnet mask selects the network bits, and the wildcard mask selects the host bits.
For an ordinary /28 subnet, four host bits give 16 total addresses. Reserving the network and broadcast addresses leaves 14 usable addresses. The /31 and /32 cases use the exceptions explained below.
How to read the answer
Start with the network address, then compare the usable range and address count with the subnet you need.
- Network address is the first address in the CIDR block.
- Broadcast address is the last address in normal IPv4 subnet notation.
- Usable range excludes network and broadcast except for /31 and /32 style cases.
Common mistakes to avoid
An IP address and its subnet address are often different. Keep the entered host address separate from the network boundary.
- Do not use this for IPv6 subnetting.
- Do not paste 10.0.5.17/28 into the IP address field. Enter the prefix separately.
- Do not assume the calculator changes any live network setting.
- The usable count is CIDR arithmetic. A cloud provider may reserve extra addresses, and a device may already use an address in the range.
- Check your router, cloud provider, or firewall rules before applying subnet plans.
Worked example: 10.0.5.17 with prefix 28
Open the Subnet Calculator and choose Small subnet /28, or enter 10.0.5.17 and prefix 28. The result is network 10.0.5.16/28.
The subnet mask is 255.255.255.240 and the wildcard mask is 0.0.0.15. The broadcast is 10.0.5.31, and the usable range is 10.0.5.17 to 10.0.5.30: 14 usable addresses.
The entered address is inside the subnet, but it is not the network address. Use the network boundary when describing the block. Check address assignments separately before giving a device an address.
Why /31 and /32 need different interpretation
For 172.16.0.8/31, this tool returns two usable addresses: 172.16.0.8 and 172.16.0.9. This follows the point-to-point /31 convention; check that both devices support that use.
A /32 represents one exact address, so 172.16.0.8/32 has one usable address. It does not describe a normal multi-device LAN.
The Broadcast field still shows the highest address in the arithmetic block. For /31 point-to-point use and /32 host routes, do not treat that label as a normal LAN broadcast destination.
Research and references
These references help check the measurements, units, limits, or safety notes used in this guide.
Worked examples for Subnet Calculator
Network 192.168.1.0, usable 192.168.1.1-192.168.1.254
16 total addresses
2 usable /31 addresses
FAQ in plain language
Do I enter the slash with the prefix?
No. Enter 10.0.5.17 in IP address and 28 in Prefix length. The prefix must be a whole number from 0 to 32.
Why is the network different from my IP address?
The network is the first address in the block. For 10.0.5.17/28 it is 10.0.5.16; your entered address is one of the addresses inside that block.
How many addresses does a /28 contain?
It contains 16 total addresses. A normal IPv4 LAN reserves the network and broadcast addresses, leaving 14 usable addresses.
Why does /31 show two usable addresses?
A /31 can use both addresses for a point-to-point link. Check the devices and link design before applying that convention.
Does the usable range show free addresses?
No. The tool does not scan a network or check DHCP leases. Existing devices, reservations, and cloud-provider rules can reduce what you can assign.
Does the tool support IPv6 or change my router?
No. It calculates IPv4 CIDR boundaries in your browser. It does not configure equipment or validate a live network.
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Keep exploring
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- All calculator and utility guidesFind more plain-language examples, formulas, mistakes, and result explanations.
- Free calculator resourcesStart here when you are not sure which calculator page fits.
Privacy and copying results
Recent answers stay visible only while you work in the current browser tab. They are not sent to a server.
Use Copy answer when you want to save the inputs and result in notes, homework, a message, or a project list. Check the units, labels, and limits before copying.