Free Online Subnet CIDR Calculator
Enter an IPv4 address in CIDR notation to instantly see its network and broadcast addresses, subnet mask, usable host range, and total address count. It helps network engineers and developers calculate usable ranges, broadcast addresses, and masks from CIDR notation.
What this calculator shows
Given an address like 192.168.1.10/24, this tool separates the network and host portions using the prefix length, then derives the network address (all host bits zeroed), the broadcast address (all host bits set to one), the subnet mask and its inverse wildcard mask, and the range of addresses actually assignable to devices on that network. This is the same math network engineers do by hand when planning IP address allocation, just done instantly and without arithmetic mistakes.
How to use this tool
Type an IPv4 address followed by a slash and a prefix length between 0 and 32, then click Calculate. Each result field has its own copy button so you can quickly grab, for example, just the subnet mask to paste into a router configuration.
Common use cases
- Planning how to split a larger network into smaller subnets for different departments or VLANs
- Checking whether a given IP address falls within a particular subnet range
- Looking up the broadcast address or usable host range for a firewall or DHCP configuration
- Converting between a prefix length like /24 and its dotted-decimal subnet mask 255.255.255.0
All calculations happen locally in your browser using standard 32-bit bitwise arithmetic — no address you enter is ever sent to a server.
How to use Subnet CIDR Calculator for related tasks
Enter an IPv4 or IPv6 network and prefix, then check the resulting range against routing and firewall assumptions before applying it.
Related tools: Base Converter, REST API Tester.
Frequently asked questions
What is CIDR notation?
CIDR (Classless Inter-Domain Routing) notation writes a network as an IP address followed by a slash and a prefix length, like 192.168.1.0/24. The prefix length says how many leading bits of the address are the fixed "network" portion — the rest identify hosts within that network.
Why do /31 and /32 networks show different usable host counts?
A /32 is a single address with no network or broadcast concept, so it has exactly 1 usable address. A /31 has only 2 addresses total, and RFC 3021 allows both to be used as host addresses for point-to-point links (no separate network/broadcast address is reserved) — every other prefix reserves the first address as the network address and the last as the broadcast address, leaving the rest usable.
Does this support IPv6?
Not currently — this calculator handles IPv4 CIDR ranges only. IPv6 subnetting uses 128-bit addresses and different conventions, which would need a separate tool.
Is my IP address sent anywhere?
No. All the subnet math runs locally in your browser using standard bitwise arithmetic — nothing you enter is sent to a server.
What does the slash number in CIDR mean?
The prefix length says how many leading bits identify the network; a larger IPv4 prefix generally means a smaller address range.