CIDR calculator
Give it a block like 10.0.0.0/22 and it returns the boundaries: the first and last address, how many hosts fit, and the mask in both notations.
Free, no signup, no daily limit. We do not store what you check here.
The two addresses you cannot use, except when you can
In a normal subnet the first address identifies the network and the last is the broadcast, so a /24 gives 254 usable hosts rather than 256. The exceptions matter in practice: a /31 has no network or broadcast address at all and provides exactly two usable addresses, which is why point-to-point links use it instead of wasting a /30. A /32 is a single host, used for loopbacks and for naming one address in a firewall rule.
Reading a prefix as a range
The number after the slash counts the bits that are fixed. Every bit you give back doubles the block, so a /23 is two /24s and a /22 is four. That arithmetic is the whole of subnetting, and it is why allow-lists are written in CIDR: one line covers a contiguous power-of-two range exactly, with no ambiguity about the edges.
Questions
Why does a /24 have 254 hosts and not 256?
The first address names the network and the last is the broadcast address, so neither can be assigned to a host. A /31 is the exception — it has neither, and both addresses are usable.
Can I enter an address that is not the network address?
Yes. 192.168.1.77/24 is understood as "the /24 containing that address", and the network address is calculated for you.
Related tools
IP in CIDR range checker
Check whether an IP address falls inside one or more CIDR blocks. Paste an allow-list and see which rule matched. Free.
Bogon and private IP checker
Check whether an address is private, reserved, loopback or documentation-only — the addresses that should never appear as a public source. Free.
IP address checker
Check whether an IP address is public, private, reserved or carrier-grade NAT. Free, no signup, IPv4 and IPv6.
When you need more than a free tool
This page answers what can be determined offline and from public DNS. The API adds live carrier and mailbox verification, a calibrated confidence score, and the full evidence trail behind every verdict.
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