CIDR Calculator
Quick Fill
?About this tool, how to use it, and FAQ▾
What is a CIDR calculator?
CIDR (Classless Inter-Domain Routing) notation describes an IP network as an address plus a prefix length, like 192.168.1.0/24. This calculator expands that notation into everything you need to plan or troubleshoot a network: the network and broadcast addresses, subnet mask and wildcard mask, total and usable host counts, the first and last usable IP, the legacy address class, and the mask in binary.
It also includes a quick reference for reserved IPv4 space — RFC 1918 private ranges, RFC 6598 shared address space (CGNAT), loopback, link-local, and the RFC 5737 documentation blocks — so you can tell at a glance whether an address is routable on the public internet.
How to use
- Enter an IPv4 address, for example 10.20.30.40.
- Pick a prefix length (/0 through /32) or an equivalent subnet mask.
- Read the results panel: network address, broadcast, usable host range, and mask details update as you type.
- Open the RFC reference to check whether the address falls in a private or reserved range.
Frequently asked questions
What does the /24 in CIDR notation mean?
The number after the slash is the prefix length: how many leading bits of the address identify the network. A /24 means the first 24 bits are the network, leaving 8 bits for hosts — 256 addresses, of which 254 are usable.
Why are two addresses in every subnet unusable?
The lowest address in a subnet is the network address and the highest is the broadcast address, so neither can be assigned to a host. The exceptions are /31 (two addresses, both usable on point-to-point links per RFC 3021) and /32 (a single host route).
Which IP ranges are private?
RFC 1918 reserves 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16 for private networks. These addresses are not routable on the public internet. 100.64.0.0/10 is shared address space used by carrier-grade NAT.