How to Use This Tool
Enter any IPv4 address — it doesn't need to be the exact network address, since the calculator will derive it for you — and pick a CIDR prefix from /0 to /32. A smaller number after the slash means a larger network with more usable hosts; a larger number means a smaller, more tightly scoped subnet. The binary breakdown above the results shows exactly which bits are fixed by your network prefix (highlighted in green) versus which bits are free to vary across hosts (shown in gray), which is the fastest way to build an intuition for how CIDR notation actually works.
The result cards below give you the network address (the first address in the range, used to identify the subnet itself), the broadcast address (the last address, reserved for sending to every host on the subnet at once), the subnet and wildcard masks in dotted-decimal form, and both the total and usable host counts.
CIDR Basics: Why Two Hosts Are "Missing"
Classless Inter-Domain Routing (CIDR) replaced the old class A/B/C system by letting the network prefix fall anywhere, not just on 8-bit boundaries. A /24 network, for instance, reserves the first 24 bits for the network portion and leaves the last 8 bits — 256 possible values — for hosts. But two of those 256 addresses are always spoken for: the very first (the network address) and the very last (the broadcast address), which is why a /24 gives you 256 total addresses but only 254 usable ones for actual devices.
- /24 — 256 total, 254 usable. The most common subnet size for small office or home networks.
- /28 — 16 total, 14 usable. A common size for small VLANs or point-of-presence segments.
- /30 — 4 total, 2 usable. Frequently used for router-to-router point-to-point links.
- /31 — a special case defined in RFC 3021: both addresses are usable (2 total, 2 usable) for point-to-point links where a network and broadcast address would otherwise waste half the range.
- /32 — a single host route, commonly used to describe exactly one address, such as a loopback interface.
Getting subnet sizing right matters most when you're planning IP allocation for a VPC, VLAN, or on-prem network segment — under-sizing means running out of addresses as you grow, while over-sizing wastes address space you may need elsewhere. If you're also sizing the server that will sit behind this subnet, our Cloud Cost Estimator and Bandwidth & RAM Calculator can help round out the planning.