Subnet Calculator

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How subnetting works

An IPv4 address is 32 bits, usually written as four numbers from 0 to 255. The prefix length (the number after the slash) says how many of those bits identify the network; the rest identify hosts within it. A /24 network keeps 24 network bits and leaves 8 host bits, giving 256 addresses. The first address is the network address and the last is the broadcast address, so 254 are usable for devices. Every extra prefix bit halves the size of the network.

CIDR to subnet mask chart

*A /31 has no network or broadcast address when used on point-to-point links (RFC 3021), so both addresses are usable.

How to use the calculator

1. Enter an address with a prefix, such as 10.20.30.40/22, or an address in the first box and a dotted mask such as 255.255.252.0 in the second.
2. Read the network, broadcast and usable host range.
3. Check the binary view to see exactly where the network bits end.
4. To plan smaller networks, choose a longer prefix in “Split into subnets” — for example split a /24 into four /26 subnets.
5. Download the subnet list as a CSV for your documentation.

Worked example

For 192.168.10.77/26 the mask is 255.255.255.192, so the last octet is split into blocks of 64. 77 falls in the block starting at 64, giving network 192.168.10.64 and broadcast 192.168.10.127. Usable hosts run from .65 to .126 — 62 addresses. The wildcard mask, used in access control lists and some routing configurations, is 0.0.0.63. Splitting the /26 into /28s gives four subnets of 14 hosts each: .64, .80, .96 and .112.

The block-size shortcut

To subnet in your head, find the “interesting” octet (the one where the mask is not 255 or 0) and subtract its mask value from 256. That is the block size. Networks start at multiples of the block size; the broadcast is one less than the next network. For a mask of 255.255.255.224 the block is 32, so networks begin at .0, .32, .64, .96 and so on.

Private and special address ranges

Planning subnets

• Size each subnet for future growth — roughly double today’s device count.
• Use /30 or /31 for router-to-router links.
• Keep related subnets contiguous so they can be summarised in routing tables.
• Document every subnet, VLAN and gateway address.
• Reserve the first or last usable address for the gateway consistently.
• Avoid overlapping ranges with VPN partners and cloud networks.

Troubleshooting with subnets

Many network problems come down to subnetting mistakes: two devices that should talk can’t because one has the wrong mask, a gateway outside the host range, or an address that is actually the network or broadcast address. Enter each device’s address and mask here and check that they share the same network address and that the gateway falls inside the usable range.

What about IPv6?

IPv6 uses 128-bit addresses, and a standard LAN subnet is a /64 — far larger than any IPv4 network — so host counting is rarely a concern. This calculator focuses on IPv4, where careful subnetting still matters because addresses are scarce.

Privacy

All calculations run in your browser. Addresses you enter are not sent anywhere.