CIDR notation is used when you’re talking about blocks of IP addresses. it’s the part of the IPv4 address — the “dotted quad” — where there’s a slash, followed by a number between 0 and 32, which represents a block of IPv4 addresses that are all related to each other. the common ones that i see all the time are /24 — which is 256 addresses, from A.B.C.0 to A.B.C.255 — and /16, which is A.B.0.0 to A.B.255.255 or 216 addresses. i’ve also seen references to /18, which is 214 addresses, but i don’t completely understand what delineates them in the /18 case — or many other cases, for that matter, it’s just that /24 and /16 have relatively visible end-points for people who don’t really understand… 😉
following that subject, i just recently encountered a block — /23 or 2562 (512) addresses — and i wondered what it was, so i looked it up, and while i was reading about it, moe came up behind me and commented that CIDR plugs are used to synchronize the œstrus of livestock animals…
ETA 190208: i have, since, encountered /11, /19, /21, and /28, which is beginning to bring about my understaning where the delineations are.