CIDR notation: reading an IP range like 192.168.1.0/24
Quick answer
CIDR notation combines a network address with a suffix, the /24 part, that states how many bits are fixed for the network portion. That suffix determines how many addresses the range actually covers: a smaller number after the slash means more addresses, a larger number means fewer. A /32 is a single address, a /24 is 256 addresses, and a /16 is 65,536 addresses.
What the slash actually means
An IPv4 address is 32 bits long. CIDR notation (Classless Inter-Domain Routing) writes an address range as an address followed by a slash and a number, and that number says how many of those 32 bits are treated as fixed, identifying the network, with the rest left free to vary across the addresses in the range. Fewer fixed bits means more bits are free to vary, which means more addresses fall inside the range. That's the whole trick to reading it: the number after the slash isn't a count of addresses, it's a count of fixed bits, and the address count follows from how many bits are left over.
Common ranges and their size
You don't need to work the bit maths out by hand to read CIDR notation day to day, it's enough to recognise the handful of suffixes that come up repeatedly:
| Notation | Fixed bits | Address count | Typically seen as |
|---|---|---|---|
| /32 | 32 | 1 | A single, specific address |
| /30 | 30 | 4 | A point-to-point link between two devices |
| /29 | 29 | 8 | A small block, sometimes seen for a handful of assigned public IPs |
| /24 | 24 | 256 | The classic "just under 256 usable" subnet, common for a single private network segment |
| /16 | 16 | 65,536 | A large private range, often used as the outer boundary a smaller /24 is carved out of |
| /8 | 8 | 16,777,216 | A huge range, mostly seen at the top of private addressing plans, not assigned whole to a single network in practice |
Two details worth keeping in mind rather than treating as full subnetting maths: the address count in that table is the size of the whole range, and in most contexts a couple of addresses within any given block are reserved (typically the very first, as the network's own identifier, and the very last, as a broadcast address), so the number of addresses actually assignable to devices is usually a little under the total. And CIDR notation isn't limited to the "clean" boundaries in the table, a /23, /27 or /25 is just as valid, this is simply a guide to the sizes you'll recognise on sight most often.
Why this notation matters practically
CIDR notation shows up constantly once you're working with servers rather than just browsing the internet, because it's the standard shorthand for describing a range rather than a single address. A firewall rule that should apply to "everything on this private network" is written as a CIDR range rather than listing every address individually. A routing table entry says which range of destinations should go a particular way, again as a CIDR range. And when you're allocating private network addressing yourself, the range you choose (and how it's carved into smaller pieces) is expressed the same way.
Being able to read /24 or /16 at a glance, and know roughly how big a range that is, saves constantly having to stop and calculate it, particularly when you're checking whether a firewall rule is scoped as tightly as you think it is, or whether two ranges you're about to use might overlap.
Where you'll see this in practice
Static IP allocations, whether a single address or a small block, are typically shown with their CIDR suffix so it's clear exactly how many addresses, and which ones, are included. See How to add additional static IPs for that in context. And the private and public ranges that make up Worldstream's own network are also described using CIDR notation, see Worldstream network architecture for the bigger picture those individual ranges sit inside.