How to read a subnet
The prefix (e.g. /24) says how many bits are the network portion; the rest address hosts. A /24 gives 256 total addresses — 254 usable, because the network and broadcast addresses are reserved. This calculator does the bit math for you and shows the full range.
What's the cheat sheet for?
Quickly map any CIDR to its netmask and host count without doing powers of two in your head — handy when you're carving up address space or reading a firewall rule.