CIDR Calculator
CIDR Calculator is a free network tool that runs directly in your browser with no account required.
About this tool
Enter an IPv4 CIDR prefix length and instantly get the dotted-decimal subnet mask, the total number of addresses in the block, and the count of usable host addresses. Handy for subnet planning, firewall rules, and cloud VPC sizing.
How to use
- Enter a prefix length from 0 to 32 in the CIDR prefix field.
- Read the Subnet mask in dotted-decimal form.
- Check Addresses for block size and Usable hosts for assignable addresses.
Frequently asked questions
Why is the usable host count two less than the total?
In a conventional subnet the lowest address identifies the network and the highest is the broadcast address, leaving 2^(32-prefix) - 2 addresses for hosts. A /24 therefore has 256 addresses but 254 usable hosts.
How are /31 and /32 handled?
They are treated as fully usable: a /31 shows 2 usable addresses, matching RFC 3021 point-to-point links, and a /32 shows 1, representing a single host route.
Does it support IPv6?
No. The calculator covers IPv4 prefixes from /0 to /32 only. IPv6 prefix math uses 128-bit addresses and different conventions.
Can I enter an address like 10.0.0.0/24?
The tool takes just the prefix length (the number after the slash). It computes mask and counts, which are the same for any network with that prefix; it does not derive network or broadcast addresses from a specific IP.
Is any network data sent while calculating?
No. The math is pure bit arithmetic in your browser, so no queries about your networks go anywhere.