Subnet Calculator
Work out the network, broadcast and usable host range for any IPv4 CIDR block, and split a network into equal subnets. Handles /31 and /32 correctly.
Subnet
- Network
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- Subnet mask
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- Wildcard mask
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- Network address
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- Broadcast address
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- First usable host
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- Last usable host
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- Mask in binary
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Related tools
All Network tools →This tool runs entirely in your browser. Nothing you enter is sent to our servers, so there is nothing for us to store or see.
This tool reads public records — DNS, WHOIS and certificate data that registries and servers publish openly. It sends nothing to the host you enter and changes nothing there. Use it on infrastructure you own or are authorised to look into.
About the Subnet Calculator
Enter an address with a prefix — 192.168.1.10/24, or with a subnet mask if that is what you have — and get the network address, the broadcast address, the usable host range, the mask, the wildcard mask and the host count.
The edge prefixes are handled properly, which is where subnet calculators most often go wrong. A /31 has no network or broadcast address: RFC 3021 defines it as a two-address point-to-point link where both addresses are usable, and a calculator that blindly subtracts two reports zero hosts or, worse, minus two. A /32 is a single host route with one address. Both are reported the way a router treats them.
The tool also says whether the address you entered is itself usable — being the network or broadcast address is the reason behind a good proportion of "why will this address not work" questions.
Two planning tools sit alongside. Split divides a network into equal subnets of a longer prefix and lists them with their ranges. Size works backwards from a host count: 500 hosts needs a /23, because a /24 gives 254 usable addresses and not 256.
How to use the Subnet Calculator
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Enter an address and prefix
Either 192.168.1.10/24 or 192.168.1.10 255.255.255.0 — both forms are accepted, including a space instead of the slash.
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Read the subnet
Network, broadcast, first and last usable host, the mask and the wildcard mask all appear together, along with the total and usable address counts.
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Check the address you entered
The tool says if it is the network or the broadcast address, which is why it cannot be assigned to a host.
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Split or size a network
Enter a longer prefix to divide the network into equal subnets, or a host count to find the smallest prefix that fits.
Frequently asked questions
Why does a /24 have 254 usable hosts and not 256?
Because two addresses in every ordinary subnet are reserved: the first is the network address, which identifies the subnet itself, and the last is the broadcast address, which reaches every host on it. Neither can be assigned to a machine, so 256 total addresses give 254 usable ones.
What is a /31 for?
A point-to-point link between two routers, where a whole /30 would waste half its addresses on a network and broadcast address nobody needs. RFC 3021 defines the /31 so both of its two addresses are usable. Many calculators still subtract two and report zero hosts, which is wrong.
What is a wildcard mask?
The bitwise inverse of the subnet mask — 0.0.0.255 where the mask is 255.255.255.0. Cisco access control lists and OSPF configuration take a wildcard mask rather than a subnet mask, and reading one as the other is a classic configuration error that silently matches the wrong traffic.
How many addresses do I need for 500 hosts?
A /23, which gives 510 usable addresses. A /24 gives 254, which is not enough. This is the arithmetic the size panel does — and it is worth remembering that any subnet you allocate should leave room for the gateway, printers, and whatever gets added next year.
Is anything sent to a server?
No. All the arithmetic runs in your browser, so nothing about your internal addressing plan is transmitted or logged anywhere.