What is CIDR?
Classless Inter-Domain Routing notation packs an IP address and the size
of its subnet into one string: 192.168.1.0/24 means
"addresses from 192.168.1.0 to 192.168.1.255." The number after the slash
is how many bits of the address identify the network; the rest identify
hosts inside it.
What's the difference between "total addresses" and "usable hosts"?
In most subnets the first address (the network) and the last (the broadcast) are reserved — neither can be assigned to a host. So a /24 has 256 total but only 254 usable. /31 and /32 are special cases: /31 supports two endpoints for point-to-point links (RFC 3021), and /32 identifies a single host.
How do I check if an address is inside a subnet?
Paste an IP in the "Check membership" field and the tool says yes or no. Useful for verifying ACL or firewall rules, or for figuring out which VPC subnet a server belongs to.
Is anything uploaded?
No. CIDR math is pure 32-bit integer arithmetic and runs in your browser.