Plan subnets: network, usable range, broadcast, and mask.
Tip: use samples, upload, copy, download, and send-to actions inside the workspace where available.
Subnet Calculator is a free, browser-based tool that helps you get the job done quickly. Plan subnets: network, usable range, broadcast, and mask. It's built for speed and privacy: Everything runs locally in your browser — your data is never uploaded to a server. No sign-up, no installs, and no daily limits.
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IPv4 Subnet Calculator: Network, broadcast, host range, mask, and class from an IP + prefix.
Open toolCIDR Calculator: Turn a CIDR block into its address range, mask, and host count.
Open toolIPv6 Calculator: Expand, compress, and derive the reverse-DNS of any IPv6 address.
Open toolThe wildcard mask is the bitwise NOT of the subnet mask. Cisco ACLs and OSPF network statements take that inverse form, where a 1-bit means “ignore this bit” — the exact opposite convention to a netmask.
300 hosts need 302 addresses once the network and broadcast addresses are reserved. The next power of two at or above that is 512 = 29, so 9 host bits are required and the prefix is 32 − 9 = /23.
| Name | Needs | Prefix | Network | Range | Broadcast | Spare | |
|---|---|---|---|---|---|---|---|
| Head office | 500 | /23 | 172.16.0.0 | 172.16.0.1 – 172.16.1.254 | 172.16.1.255 | 10 | |
| Warehouse | 120 | /25 | 172.16.2.0 | 172.16.2.1 – 172.16.2.126 | 172.16.2.127 | 6 | |
| Guest Wi-Fi | 60 | /26 | 172.16.2.128 | 172.16.2.129 – 172.16.2.190 | 172.16.2.191 | 2 | |
| Servers | 25 | /27 | 172.16.2.192 | 172.16.2.193 – 172.16.2.222 | 172.16.2.223 | 5 | |
| Router link | 2 | /31 | 172.16.2.224 | 172.16.2.224 – 172.16.2.225 | 172.16.2.225 | 0 |
How this was allocated: requirements are sorted largest-first, each is rounded up to the next power of two, and blocks are placed at the first address that is a multiple of their own size. That alignment rule is what CIDR requires — a /26 can only start at .0, .64, .128 or .192 — and allocating big blocks first is what prevents small subnets from fragmenting the space and stranding the large ones.
